The Terramaster box doesn't support hardware RAID so I think you created a "Spanned Volume" over the two drives. This uses the Logical Disk Manager (LDM) capabilities which are not natively supported on Linux, although there is a tool called "ldmtool" which can be used to create device-mapper entries and mount volumes. However, LE does not bundle ldmtool into our images, we do not support package management, and we are intentionally missing all the software RAID and device-mapper bits (so we don't have to support it).
This community image from sky42 has the LVM bits you need LE 12.0 added lvm2, luks (dm-crypt, veracrypt), mdraid, ext4 encryption .. but not ldmtool AFAICT. You can try copying the ldmtool binary direct from an Ubuntu image; if you're lucky it's an independent binary that's not linked against a bunch of libraries. If you're not, it is, and it needs to be compiled properly for LE use. That image also has the bits needed to make a Linux compatible software RAID (JBOD) setup; though that will require you to move data off the disks to create a new JBOD array before copying data back.
Of course, even if you make something work, it's not supported by staff.