The Disk you inserted was not readable by this computer

I have a 15" Macbook Pro with Retina Display. I bought it when it first cam out in the summer. I've been keeping a regular time machine back up of my system ever since. I use a Seagate 3TB GoFlex Desk drive with the Thunderbolt adapter. I haven't had any issues so far and back up's have worked fine.

Yesterday I decided that I wanted to wipe my Mac's internal drive, re-install OS X Mountain Lion, and start fresh. So I forced one more back up on the Seagate, and then proceeded to reformat my mac.

After I reinstalled Mountain Lion and the updates, I plugged in my Seagate/Time Machine Back-Up drive, and got the error, "The disk you inserted was not readable by this computer".

I searched for answers and found some peope suggesting to reset my PRAM and SMC. So I did that, and yet the problem persists. I can see the disk in my Disk Utility, but the options to repair or verify are greyed out.

I did have a seperate back-up drive with my most important documents. However there was some recent work that I had only a time-machine back up of. There must be a solution as I find it unlikely there is something wrong with the drive. Again, the drive was working fine just a day earlier, and now after the reformat, it's unreadable.

Is it possible that the reformat got rid of a driver that gave my mac the ability to read the drive?

Any advice is greatly appreciated.

Sincerely,

- Matt