What 3 TB External Hard Drive is BEST and Compatible with 64 bit Intel Core i5 iMac 11,1?

I have been looking for a while, and haven't been able to find a straight answer.

I have an Apple iMac Intel Core i5 2.66 GHz 64-bit 27-inch Desktop (iMac11,1).

I am looking for a 3 Terabyte External Hard Drive that is compatible with this mac.

I had bought a Seagate FreeAgent Go Flex Desk External Drive:

http://www.seagate.com/www/en-us/products/external/external-hard-drive/desktop-h ard-drive/

I plugged it in, I read the quick installation guide. I proceeded to do what they suggested with the Disk Utility> Partition process.

I copied one file, it worked. I tried to copy a lot of files at once, it didn't work.

I returned it to BestBuy, where I got it from and ask the guy from the GeekSquad to recommend something else. He said the reason this one didn't work is because it was compatible with 32-bit equipment and my mac is 64-bit.

I asked him to help me find one in the store that would be compatible with my 64-bit iMac. We looked at a 2 GB G-Drive, and at a 3 TB WD MyBook, and they stated that they where compatible with both 32-bit and 64-bit Windows computers, but they didn't say anything about 64-bit Mac.

What to do ??? I go back home, researched on the internet, and found nothing that adresses this issue. I thought I'd post here and see if anyone out there has a suggestion for a 3 TB External Hard Drive for my 64-bit Apple iMac Intel Core i5 2.66 GHz 27-inch Desktop (iMac11,1).

Thank you,

MW