iMac issues with hard drive(s)

I have a 2 year old iMac, my external hd that I store my digital media on crashed. I reformatted it, of course loosing all of the data. Within 24 hours my iMac was unresponsive. I restored the computer using my time machine backup. It immediately revevert to the nonresponsive iMac. I have the 1TB Seagate hard drive refered to in the "quasi-recall" but my specific serial number doesn't apply for the free fix. I am not sure how, but both drives now will not run my operating system without freezing up. I have repeated this step, trying to load and run my mac from my external thunderbolt hd, in which I thought I had repaired. Do I need a new hard drive, and if so.. what interface do you think it is, and should I go with another stupid Seagate?