sd is for (originally) scsi disk devices, however it seems to now refer to removable devices in general and SATA devices. and the letter is just the number of the device, starting at a, with the number indicating the partition.

What does SD stand for in Linux?

The first thing that you need to know is there is no C drive or E drive in Linux. You will see something like /dev/sda, /dev/sdb, /dev/sdc, … etc. instead. The dev is short for device. The sd was short for Small Computer System Interface (SCSI) mass-storage driver.

What does SD in SDA stand for Linux?

The term sd stands for SCSI disk, that is to say, it means Small Computer System Interface disk. So, sda means the first SCSI hard disk. Likewise,/hda, the individual partition in the disk takes names as sda1, sda2, etc.. The active partition is indicated by an * in the middle column.