I think this may be some sort of bug, I just tried using that like 10 minutes ago and it gave me the same error, but my drive is fine.
But I found one more strange thng
I checked the total sectors in my HDD using Other tools like hdd tune and drive xml
which it is = 976764911
But on Seagate site it states that for the same model ST3500418AS it should be = 976773168
Can u also check that in ur case?