Trying to recover data from RAID-1 disk.

If you are certain that the drives were setup in RAID 1 Mirroring and not RAID 0 Stripped...?

Then try recovering data from the other drive in the array.

If they were in RAID 0 Stripped, then you need to replace that drive back into the Terra Master with the other drive and try to recover the data.

From your screenshot, there appears to be four partitions on the drive which apparently are not accessible by macOS or they would have been mounted. Some NAS systems use a Linux or BSD OS which can use one of multiple file systems and/or utilities to allow for RAID....none of which macOS can understand natively. The NAS is using a network protocol (most likely SMB) to allow other devices to access the shares so it doesn't matter to your devices how the NAS itself configures those drives & file systems. Without knowing how the NAS internals work, no one here can help you.

You are going to have to contact TerraMaster support for assistance. Unfortunately they believe in hiding all information from the public so I cannot even access the user guide for any of their NAS systems. That should be a huge red flag. I guess you can try posting on the TerraMaster forum as well.

Good luck.

If the NAS is using Linux and the BTRFS file system for the RAID, then connecting the drive to a Linux system may allow you to access the data. However, if the NAS uses any other setup for the RAID features, then the volume may not be accessible without some Linux & command line knowledge.

If you have access to a Windows PC, then you could try booting that PC using a Knoppix Linux USB stick to see if you can view the contents of one of those partitions. Knoppix has access to the BTRFS file system and many others. Knoppix can also boot older non-USB-C Macs as well. Macs and some PCs may be a bit tricky trying to boot Knoppix externally, but it will boot most systems.

This is a bit of a long shot.

FYI, you should also have a backup of any important data on that NAS in addition to backups of your computer and all other external media (including the cloud). Plus RAID is not a backup, although a backup device can utilize RAID to try to ensure the backup is available at all times even when one of the drives fails & needs to be replaced (assuming any RAID type other than 0).

In no particular order…

If the volume was RAID-0 and not RAID-1, then you need both storage devices working, or roughly half your data is gone.

Some of those TerraMaster boxes are running a Linux variant, and the file system used can potentially be BTRFS or EXT4, neither of which are supported by Mac.

TerraMaster will know the details of their hardware better, and you’ll likely need to identify the model to them. They may have a firmware recovery path available, too.

Trying to access a RAID array from a different system can potentially corrupt the RAID storage volume, too. Particularly if the other system tries to write to the storage.

If the data is valuable, maybe work with a data recovery service? Or with the NAS vendor, if less valuable.