As a victim of a few attempts with Linux I have found that the easiest way to return a hard drive to working condition is using
aefdisk a replacement for the msdos fdisk. This will kill any errant non dos boot partitions that linux has a habit of leaving around that can't be removed with fdisk. Be careful though, the killall option does just that
When you say you used the seagate installer to fix the problem what did it do? I am guessing in that it added a "dos extender" boot file to try and access the non dos partitions. This started the ball down the slippery slope
Let me know how you get on.
MAx