NTFS disks have proven notoriously problematic with Nucleus/ROCK. Did you update the ROCK prior the January 22?
New updates of ROCK use a stricter driver for NTFS, which is much safer in avoiding corruption. However, to achieve this it refuses to use disks that have any issues in the NTFS format.
If you ever unplugged the disk from the ROCK without shutting it down first, or unplugged it from a Windows PC without clicking the proper “remove safely” button, this would cause the disk to be flagged as “unsafely removed” and then new ROCK versions won’t mount it again to prevent file system damage, until it is checked and cleaned in Windows.
In your case, since it is just a backup disk and doesn’t have loads of music on it, I would just reformat it with the exFAT disk format instead of NTFS. The exFAT format is what Roon recommends for external disks that are attached to ROCK.
In Windows, this is done like this:
See the longer story here: