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
If you are getting problem with your partition manager tool then download GParted tool. Since this tool is designed for creation and management of partition hope this tool rescue you from the problem. http://www.downloadgparted.com
I realise that I am responding to a very old post, but the disparity in the number of sectors (976764911 vs 976773168) could be due to BIOS stealing sectors from the end of the drive in order to store a backup copy of itself in a Host Protected Area. Gigabyte motherboards with an Xpress Recovery BIOS will do this.