Same issue here — several different drives. Disk Utility does see the volume, but I'm unable to mount it even then.

I've found some additional steps that at least will get my drives mounted: The mounting issue seems to happen after a restart If, after restart, I log into one account, the drive(s) won't mount Then, if I log into another user account on the machine, the drives do mount. I can then log out of the second account and back into the first account, and the drives will mount (or stay mounted). If I do this same set of steps but swap the accounts, the same thing happens — so I don't think it's a user account problem.

Awesome find — that worked for me as well. Thanks!