There are unresolved issues trying to use Windows 7's built-in backup feature with external USB HDDs larger than 2TB. It is unknown to me whether or not this is fixed in Windows 8. But it's pretty shitty of MS not to fix those bugs for those of us still on Windows 7.
Just like the non-C-drive defrag issues, that take up all of your RAM + pagefile space in Windows 7. MS has never issued a patch for that either.
It would be nice if MS could release an OS where everything "just works", for once.