External Time Machine Drive won't mount or initialize

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.

I recently needed to do a clean installation of OSX and downgraded from El Capitan to Yosemite, because at this time Adobe programs work better with Yosemite.

After doing a clean installation of 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 formatted by OSX for Time Machine. There are no partitions on the drive, it was solely dedicated for use as a Time Machine backup drive.

I tried restarting with the drives connected, disconnected, and even swapped the drives into opposite Thunderbolt bays but the TM drive still fails to mount.

Any ideas?