question: What happens to the space on a hard disk that we're unable to use? For example in a 1TB hard disk, we get about 930GB of usable memory, what happens to the other 70GB?
comment: There are 2 values of data. TB (terabyte) and TiB (tebibyte). the "ra" ones are using multiplies of 1000. the "bi" ones are using multiplies of 1024. I will do some math for you: 1 TB=1000\^4B=1000000000000 B 1 TiB=1024\^4B=1099511627776 B 1 TB=1000000000000/1099511627776 TiB=0.931322575 TiB Hard disc that have 1 TB have 0.93 TiB. There goes your 70 GiB
passages: Section::::Capacity.
  
  The capacity of a hard disk drive, as reported by an operating system to the end user, is smaller than the amount stated by the manufacturer for several reasons: the operating system using some space, use of some space for data redundancy, and space use for file system structures. Also the difference in capacity reported in SI decimal prefixed units vs. binary prefixes can lead to a false impression of missing capacity.
  
  Section::::Capacity.:Calculation.
   Approximately 7 GB of a 60 GB drive is reserved for system use (4 GB of that portion is reserved for game title caching and other hard drive specific elements in games that support the hard drive, and an additional 2 GB is reserved for use by the Xbox 360 backwards-compatibility software). This leaves just under 54 GB of free space, rounded down to 53 GB in the dashboard, for saving game files, Xbox Live Arcade downloadable content, and media files (such as music and video). Similar figures apply to other hard disk drive sizes.
   In modern HDDs, spare capacity for defect management is not included in the published capacity; however, in many early HDDs a certain number of sectors were reserved as spares, thereby reducing the capacity available to the operating system.
   (3,600 s × 300,000 bit/s) / (8×1024×1024) requires around 128 MB of storage. 
   Between 1980 and 2000, personal computer hard disk sizes increased from about 10 megabytes (10 bytes) to over 100 gigabytes (10 bytes). A 100-gigabyte disk could store the favorite color of all of Earth's seven billion inhabitants without using data compression (storing 14 bytes times 7 billion inhabitants would equal 98 GB used). But what about a dictionary-on-disk storing all possible passwords containing up to 40 characters? Assuming each character equals one byte, there are about 2 such passwords, which is about 2 × 10. In his paper "Computational capacity of the universe", Seth Lloyd points out that if every particle in the universe could be used as part of a huge computer, it could store only about 10 bits, less than one millionth of the size such a dictionary would require. However, storing information on hard disk and computing it are very different functions. On the one hand storage currently has limitations as stated, but computational speed is a different matter. It is quite conceivable that the stated limitations regarding storage have no bearing on the limitations of actual computational capacity, especially if the current research into quantum computers results in a "quantum leap" (but see "holographic principle").
   Hard drives are often measured in terms of cost per bit. For example, the first commercial hard drive, IBM's RAMAC in 1957, supplied 3.75 MB for $34,500, or $9,200 per megabyte. In 1989, a 40 MB hard drive cost $1200, or $30/MB. And in 2018, 4 Tb drives sold for $75, or 1.9¢/GB, an improvement of 1.5 million since 1989 and 520 million since the RAMAC. This is without adjusting for inflation, which increased prices nine-fold from 1956 to 2018.
   Persistent disks are independent of the virtual machines and outlive an instance's lifespan. All information stored on the persistent disks is encrypted before being written to physical media, and the keys are tightly controlled by Google.
  
  Each instance can attach only a limited amount of total persistent disk space (one can have up to 64 TB on most instances) and a limited number of individual persistent disks (one can attach up to 16 independent persistent disks to most instances).
  
  Section::::Resources.:Snapshot.
   File systems can be used on numerous different types of storage devices that use different kinds of media. As of 2019, hard disk drives have been key storage devices and are projected to remain so for the foreseeable future. Other kinds of media that are used include SSDs, magnetic tapes, and optical discs. In some cases, such as with tmpfs, the computer's main memory (random-access memory, RAM) is used to create a temporary file system for short-term use.
   Below are several digital images illustrating data degradation, all consist of 326,272 bits. The original photo is displayed on the left. In the next image to the right, a single bit was changed from 0 to 1. In the next two images, two and three bits were flipped. On Linux systems, the binary difference between files can be revealed using command (e.g. ).
  
  Section::::In RAM.
   Examples of partition mapping scheme include Master boot record (MBR) and GUID Partition Table (GPT). Examples of data structures stored on disk to retrieve files include the File Allocation Table (FAT) in the DOS file system and inodes in many UNIX file systems, as well as other operating system data structures (also known as metadata). As a consequence, not all the space on an HDD is available for user files, but this system overhead is usually small compared with user data.
  
  Section::::Capacity.:Units.
   Software reports hard disk drive or memory capacity in different forms using either decimal or binary prefixes. The Microsoft Windows family of operating systems uses the binary convention when reporting storage capacity, so an HDD offered by its manufacturer as a 1 TB drive is reported by these operating systems as a 931 GB HDD. Mac OS X 10.6 ("Snow Leopard") uses decimal convention when reporting HDD capacity. The default behavior of the command-line utility on Linux is to report the HDD capacity as a number of 1024-byte units.
   Data is stored on a hard drive in a series of logical blocks. Each block is delimited by markers identifying its start and end, error detecting and correcting information, and space between blocks to allow for minor timing variations. These blocks often contained 512 bytes of usable data, but other sizes have been used. As drive density increased, an initiative known as Advanced Format extended the block size to 4096 bytes of usable data, with a resulting significant reduction in the amount of disk space used for block headers, error checking data, and spacing.
   Section::::Technology.:Error rates and handling.
  
  Modern drives make extensive use of error correction codes (ECCs), particularly Reed–Solomon error correction. These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity. For example, a typical 1 TB hard disk with 512-byte sectors provides additional capacity of about 93 GB for the ECC data.
   In physical terms, most computer files are stored on some type of data storage device. For example, most operating systems store files on a hard disk. Hard disks have been the ubiquitous form of non-volatile storage since the early 1960s. Where files contain only temporary information, they may be stored in RAM. Computer files can be also stored on other media in some cases, such as magnetic tapes, compact discs, Digital Versatile Discs, Zip drives, USB flash drives, etc. The use of solid state drives is also beginning to rival the hard disk drive.
   The most common physical sector size for harddisks today is 512 bytes, but there have been hard disks with 520 bytes per sector as well for non-IBM compatible machines. In 2005 some Seagate custom hard disks used sector sizes of 1024 bytes per sector. Advanced Format hard disks use 4096 bytes per physical sector (4Kn) since 2010, but will also be able to emulate 512 byte sectors (512e) for a transitional period.
  
  Magneto-optical drives use sector sizes of 512 and 1024 bytes on 5.25-inch drives and 512 and 2048 bytes on 3.5-inch drives.
   A number of technologies are attempting to surpass the densities of existing media.
  
  IBM aimed to commercialize their Millipede memory system at 1 Tbit/in in 2007 but development appears to be moribund. A newer IBM technology, racetrack memory, uses an array of many small nanoscopic wires arranged in 3D, each holding numerous bits to improve density. Although exact numbers have not been mentioned, IBM news articles talk of "100 times" increases.
   Today, the hard drive industry continues to use decimal prefixes for drive capacity (as well as for transfer rate). For example, a "300 GB" hard drive offers slightly more than , or , bytes, not (which would be about ). Operating systems such as Microsoft Windows that display hard drive sizes using the customary binary prefix "GB" (as it is used for RAM) would display this as "279.4 GB" (meaning bytes, or ). On the other hand, macOS has since version 10.6 shown hard drive size using decimal prefixes (thus matching the drive makers' packaging). (Previous versions of Mac OS X used binary prefixes.)
   All converted files are available and writable in the default subvolume of the Btrfs. A sparse file holding all of the references to the original ext2/3/4 filesystem is created in a separate subvolume, which is mountable on its own as a read-only disk image, allowing both original and converted file systems to be accessed at the same time. Deleting this sparse file frees up the space and makes the conversion permanent.
   In theory, the extent allocation tree makes a conventional free-space bitmap unnecessary because the extent allocation tree acts as a B-tree version of a BSP tree. In practice, however, an in-memory red-black tree of page-sized bitmaps is used to speed up allocations. These bitmaps are persisted to disk (starting in Linux 2.6.37, via the codice_15 mount option) as special extents that are exempt from checksumming and copy-on-write. The extent items tracking these extents are stored in the root tree.
  
  Section::::Design.:Checksum tree and scrubbing.
   This example concerns a Windows XP system with two physical hard disks. The first hard disk has two partitions, the second has only one. The first partition of the first hard disk contains the operating system. Mount points have been left at defaults.
  
  In this example, 
  
  BULLET::::- "C:", "D:", and "E:" are volumes.
  
  BULLET::::- Hard Disk 1 and Hard Disk 2 are physical disks.
  
  BULLET::::- Any of these can be called a "drive".
  
  Section::::Nomenclature of volumes.
   BULLET::::- "giga" ("G"): A multiplier equal to 1,073,741,824 (2 or K, where K = 1024).
   With recently demonstrated technology using DNA computing for storage, one yottabyte of capacity would require a volume between 0.003 and 1 cubic metre, depending on number of redundant backup copies desired and the storage density: "Our genetic code packs billions of gigabytes into a single gram". DNA is much more advanced technology than microSDXC cards (for this application) and accompanied by uncertain costs, but this suggests potential information density.
   Digital object memory
  
  A digital object memory (DOMe) is a digital storage space intended to keep permanently all related information about a concrete physical object instance that is collected during the lifespan of this object and thus forms a basic building block for the Internet of Things (IoT) by connecting digital information with physical objects.
   Digital disk drives are block storage devices. Each disk is divided into logical blocks (collection of sectors). Blocks are addressed using their logical block addresses (LBA). Read from or writing to disk happens at the granularity of blocks.
  
  Originally the disk capacity was quite low and has been improved in one of several ways. Improvements in mechanical design and manufacture allowed smaller and more precise heads, meaning that more tracks could be stored on each of the disks. Advancements in data compression methods permitted more information to be stored in each of the individual sectors.
   Also as a vdev cannot be shrunk in size, it is common to set aside a small amount of unused space (for example 1-2GB on a multi-TB disk), so that if a disk needs replacing, it is possible to allow for slight manufacturing variances and replace it with another disk of the same "nominal" capacity but slightly smaller "actual" capacity.
  
  Section::::Terminology and storage structure.:Cache devices.
  
presuppositions: Hard disk is only measured in one way. There is space on a hard disk that is not usable.
corrections: Hard disk is measurable in 2 values of data, not one. All the space on a hard disk is usable, but there are two different units of measure.
raw_presuppositions: Hard disk is only measured in one way. There is space on a hard disk that is not usable. There is space on a hard disk that is not usable.
raw_corrections: Hard disk is measurable in 2 values of data, not one. All the space on a hard disk is usable, but there are two different units of measure. All the space on a hard disk is usable, but there are two different units of measure.