[Corso]

Frankly, it's most likely failing. Transferring large files can eat up a HDD's lifespan. If I were you, I'd look into Spinrite or getting the Seagate Hard Disk Utility which analyzes the current state of the drive (good, bad, failing so forth). Then attempt repairs if needed.

Also, when it freezes or slows down to a crawl - Check your Event Viewer in your Computer Management. Might show some insight on why, what errors the Windows System is receiving and logging and so forth.

[Corso]

Oh second time seeing post and thought to myself, see if you can see the device clearly in your BIOS and maybe try running it on uBuntu (you can run a uBuntu false install right off disk without actually installing it). That will tell you a lot of the items Spinrite would just not do much good on repairs in comparison.