Support automatic USB disk detection for systems with multiple connected USB disks

[andreashaerter]

By default, the script is simply using the first partition on the first USB disk it is able to find via /dev/disk/by-path/. No worries: existing drives not used for backups won't be destroyed as the decryption will fail. But this automatism presumes that only one USB disk is connected during the script run.

Defining a UUID will work if there are more than one (e.g. when it is not feasible in your environment to just have one disk connected simultaneously). There is the -d parameter wich allows to define a UUID of the target partition to decrypt which will be used to search it in /dev/disk/by-uuid/ (you might use blkid /dev/sdX1 to determine the UUID).

This could (and should) be improved:

• The script should try all existing USB disks (not only the first).
• The -d parameter should be an Array and all UUIDs should be tried in the defined order. It should be documented that this does support non-USB-disks, too (as a side note)