Same problem with a Seagate 3TB Go Flex. It hasn't backed up anything with Time Machine for a couple of months--so I didn't bother with it. Attaching it to my iMac made the machine too sluggish to be usable. But now I see that the darn drive is apparently no longer formatted as "Mac OS Extended (Journaled)" or whatever. I'm wondering what the best way is of getting it formatted right.
Somewhere it says that reformatting will cause you to lose all of the backups that are now on the drive from the good old days when it was formatted as Mac OS etc. This is a lose-lose situation. And you say yours was a Mac drive to begin with?
I'm going to sell this baby for fifty bucks and pick up a 1TB WD Passport drive (with USB3.0) for $25 more. I've never had this kind of trouble with a WD Passport or Elements drive.
Is there any way to reformat the Seagate back to Mac OS Extended and in the process not lose backed- up files that were previously loaded onto it?