One option is the Seagate Backup Plus product line, either the portable or desktop versions. They come with USB3, but if USB2 speeds on the Mac Pro get you down, you can buy a $25 adaptor for FireWire 800. Seagate also promises a Thunderbolt adaptor for thr portable versions, but I do not see it for sale yet.
P. S. It is usually best to skip the special "Mac OS versions": just reformat the "regular PC" version.