When starting Tails, set up an administration password.

Choose Applications ▸ System Tools ▸ Root Terminal.

Execute the following command:

lsblk

The output is a list of the storage devices and partitions on the system. For example:

  NAME                   MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
  loop0                    7:0    0  1.2G  1 loop  /lib/live/mount/rootfs/filesystem.squashfs
  sda                      8:0    1    7G  0 disk
  ├─sda1                   8:1    1    4G  0 part  /lib/live/mount/medium
  └─sda2                   8:2    1    3G  0 part
    └─TailsData_unlocked 253:0    0    3G  0 crypt /run/nosymfollow/live/persistence/TailsData_un...
  zram0                  254:0    0  2.8G  0 disk  [SWAP]

Plug in your encrypted volume. Keep the encryption locked.

Execute the same command again:

lsblk

Your encrypted volume appears as a new device with a list of partitions. Check that the partition size corresponds to your encrypted volume.

  NAME                   MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
  loop0                    7:0    0  1.2G  1 loop  /lib/live/mount/rootfs/filesystem.squashfs
  sda                      8:0    1    7G  0 disk
  ├─sda1                   8:1    1    4G  0 part  /lib/live/mount/medium
  └─sda2                   8:2    1    3G  0 part
    └─TailsData_unlocked 253:0    0    3G  0 crypt /run/nosymfollow/live/persistence/TailsData_un...
  sdb                      8:0    1    7G  0 disk
  └─sdb1                   8:2    1    7G  0 part
  zram0                  254:0    0  2.8G  0 disk  [SWAP]

Take note of the partition name of your encrypted volume. In this example, the new device in the list is sdb and the encrypted volume is in the partition sdb1. Yours might be different.