I'm currently copying a bunch of files over from a Seagate 1tb external hard drive to a Toshiba 2tb and for some reason on the Seagate it says there's 846 GB of data, but when copied over it's only 681 GB. Any ideas why this may be? I check every folder and everything is there, I'm also using a macbook pro.
I'm not a Mac person, but if this was a PC, it could be caused by differing cluster sizes between the two drives, but that much difference is hard to explain unless you have a gizillion very small files.
I would advise running a file compare program that check the content of every file. I like BeyondCompare on the PC (don't know if it runs on Mac. Make sure you set the option to compare file contents. It will take a LONG time for this much data.