Mt. Everything is similar to SCSI Probe. Something like that, or even Drive Setup, will enable you to tell if the Mac can actually see the drive (before attempting to mount it).
If the drive is not seen, there is probably a hardware/connection type issue. And if nothing obviously wrong, it is probably not worth spending too much time on.
If the drive can be seen, but not mounted, then a disk repair tool may recover the situation. Disk Warrior, Norton Utilities Disk Doctor, or even Disk First Aid are worth a try.