I have an external ssd with ubuntu-mate installed.
I get an error message trying to mount it
(Unable to mount encrypted, Operation cancelled)
Funny thing is, I always get the error message however on some computers it works
(disk is decrypted and mounted) and on others it does not. 
What tool was used to encrypt this SSD? Could you show us a screengrab of what this SSD looks like as viewed by Disks (formerly the GNOME Disk Utility)?
Just a normal UM installation with luks disk encryption enabled during installation. Nothing custom or fancy.
Although I can confirm it 'does' get unlocked, its not getting mounted on some machines. (not visible in caja 
)
You can reproduce this by installing UM onto an external SSD
and then plug it in another computer with UM installed.