The sg driver allows users to send SCSI commands to SCSI aware devices - for example scan for disks. Once the scan has completed the SCSI disk drive (sd) can be initialised providing block level access to the media - typically only performing 'SCSI READ' and 'SCSI WRITE' commands.
In order to retrieve a list of SCSI devices we can use the 'lsscsi' command (I prefer this than the normal lsblk since it conveniently provides additional information such as the SCSI bus, channel and LUN numbers etc.)
lsscsi
[0:0:0:0] disk ATA WDC WD1111111-111 1A01 /dev/sda
[1:0:0:0] cd/dvd HL-DT-ST DVD-RAM GHC0N MA01 /dev/sr0
[4:0:0:0] disk Generic- Multi-Card 1.00 /dev/sdb
The sg driver is commonly used to interact with devices such as scanners and CD-ROM drives.
Interestingly you might have noticed that almost all disk types use the sd driver - but quite often these disks might be ATA or SATA drives. The SCSI driver is used to abstract the many different disk types - so in fact it is compatible with a whole host of different disk types.
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