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==== Disk basics ====

//source:// [[https://wiki.restarters.net/UEFI_and_GPT|wiki.restarters.net]]

A hard disk consists of one or more platters with a magnetic recording layer on one or both surfaces of each platter. Data is recorded in a series of concentric circles or "tracks", each comprising a number of blocks of data known as "sectors". A sector (traditionally, 512 bytes but now 4096 bytes or larger) is the minimum unit that can be read or written. The set of corresponding tracks of a given radius on all the surfaces is known as a "cylinder". Each surface has its own read/write head but the heads all move together, so a cylinder is the totality of storage that can be read or written without moving the heads to a different set of tracks.

Originally, any sector was identified by its cylinder, its track (or read/write head) within the cylinder and its sector within the track. The start and the end of a partition were defined by the cylinder/track/sector of its first and last sectors, but with increasing disk sizes this became impracticable. Besides, for many years now disks have optimised storage capacity by placing more sectors per track on the outer tracks where the track circumference is greater. Hence, partitions are now normally specified by the absolute start and end sector numbers.This is known as Logical Block Addressing or LBA.

=== MBR ===
The MBR is located at the end of the first sector of a disk. The remainder of that sector contains a bootstrap program which the BIOS loads and executes as the first stage of the boot process.

=== GPT ===
GPT reserves the first sector of a disk for a "protective MBR" so that programs which only understand MBR don't think the disk is unformatted and corrupt it. This MBR defines a single partition comprising the whole disk, or as much of it as it can address.

Additionally, a GPT-formatted disk contains contains 2 almost identical copies of the GPT header, one starting at the second sector (to avoid the protective MBR) and the other ending on the last sector of the disk. These headers can define up to 128 partitions using 64 bit addressing and hence allowing for disk sizes 16 billion times larger than MBR allows.

==== Diszkek keresése ====
<code bash>
  echo "- - -" > /sys/class/scsi_host/host0/scan
</code>

**Egy diszk újraszkennelése:**

<code bash>
  echo "1" > /sys/class/block/sdX/device/rescan
</code>