Time Machine Drive Won't Mount

I have two Seagate 3TB External Thunderbolt drives attached to my 27 iMac - one I use as a regular storage disk and the other I use as a Time Machine Drive for both my Internal HD and the storage 3TB.

I recently needed to do a clean installation of OSX (due to a power issue - unpredictable power downs during normal or heavy computer use) and downgraded from El Capitan to Yosemite, because at this time Adobe programs work better with Yosemite.

After doing a clean installation of OS X Yosemite, the first storage drive is fine, but the other Time Machine drive fails to mount upon startup. The options are to initialize, ignore or eject. Initializing takes me to Disk Utility, but the options to verify or repair disk are greyed out. The details below read that the disk is:

• not mounted
• partition type: 0xEE
• capacity 375.07 GB

Right-clicking, I can mount or eject, mounting does nothing.

I downloaded the Paragon NTFS drivers from the Seagate website, but that did not help. The drive was formatted for Mac OS Extended (Journaled) or so I think when a disk is used by OSX for Time Machine. There are no partitions that I set up on the drive, it was solely dedicated for use as a Time Machine backup drive.

I tried restarting with the drives connected, and disconnected. I even swapped the drives into the other Thunderbolt bay, and the bad TM drive still didn't mount while the storage drive is fine.

Any ideas?