The danger with USB hard disk drives is that when you have more than one plugged into your workstation, the device name assigned to it by the operating system might not be consistent between reboots. i.e. /dev/sdb1 and /dev/sdb2 might swap places. Potential disaster if you rsync data from one to the other on a periodic basis.
If permanently mounting usb hard disks, it’s much safer to mount according to the UUID of the disk instead of the device name assigned by the OS.
If you change to root using sudo su – and cd into /dev/disk you’ll see that there are multiple links in there, organised into different folders. The unique unit id is written in /dev/disk/by-uuid and links the device name to the unique id.
You can see what device name is mounted where using df -h. Then use the output of ls -al of /dev/dsk/by-uuid to correlate uuid to filesystem mount. There’s probably other ways to match filesystem to uuid but this is quick and easy enough to do.
Note that I’ve also taken the liberty of piping the commands through grep to reduce output, just showing me what I want to know, i.e. the uuid’s mounted to devices named /sda1, /sda2, /sdb1 etc.
Once you’re confident you know what UUID is what disk, then you can permanently mount the disk or disks that are permanent fixtures by creating a mount point in the filesystem and adding a line to /etc/fstab
finally, mount -a will pick up the UUID and mount it into the mount point.