Start at the beginning of this thread to see how complex it might get..
https://www.raspberrypi.org/forums/view ... 8#p1515778
Generally speaking, anything (filesystem-wise) ubuntu can see, the pi can also - ie: ext2,3,4. I haven't tried xfs,btrfs,zfs on the pi (nor encrypted, aka luks) so lets assume that's not the case or you'd have said? I do know it supports lvm2 with little effort.
Assuming no hardware problems (above link) and no bizarre file systems (above) then when you plug that usb caddy into ubuntu it should (possibly automount - I use Mint). Post output of this..
..before inserting the usb cable. Repeat after inserting it. Post both else nobody can help further. I'll be away until next week so help others on the forum help you iyswim