I’ve got an external G-DOCK, a thunderbolt hard drive bay with VeraCrypt enabled. I want to use it for my Mac’s Time Machine backups.

According to Apple, Time Machine supports all Mac OS Extended (Journaled) formats and Xsan formats. In my case, the container is mounted exactly as Mac OS Extended(Journaled), but Time Machine does not allow me to select this volume.

To solve this problem, all we need to do is manually add the partition, fire up a terminal window and type:

sudo tmutil setdestination -a /Volume/NameOfYourVolume

You could check your Time Machine destinations with tmutil destinationinfo

❯ tmutil destinationinfo
Name          : File Vault
Kind          : Local
Mount Point   : /Volumes/File Vault
ID            : F3ECB098-25B8-40E9-92D8-D39CF4C6AB6B

References: Time Machine backups to a Veracrypt volume?