Given that the mac is not mounting the drive set, that usually means an issue with both drives or directory. Did you try selecting the mirror set in Disk utility and running disk repair?
When mirrored, you should be able to mount one of the drives and see all the data. Mirroring just duplicates the writes, i.e. data is written to both drives at the same time. So you would need one enclosure or, as suggested, a disk dock type device to plug the bare drive into and try one disk then the other alone. The one that won't mount is corrupted. If they both won't mount, then both are corrupted and you will need to resort to a data recovery tool or service, or to your backup data and restore.
It is not unheard of for an OS or app problem to end up corrupting both pair of a mirror set, but its rare.