Disk utility will not allow me to resize a partition on an unencrypted external hard drive I want to back up a copy of Windows onto a new partition of external hard drive currently formatted in exFAT. I don't know how to use Windows, so I want to use my Mac (mid-2009 MacBook Pro, running Mavericks, which it shouldn't be&mdash;it was built for Snow Leopard) to repartition the disk. 

But it doesn't seem to let me. The name, format, volume size, and "+" are all grayed out, and the little handle that is supposed to let me resize the partition so as to add a new one in the remaining space is not showing up. (Actually, I have two such hard drives. In one of them, the "minus" option isn't available either.) Neither shows up as encrypted; other answers to similar problems tend to depend on the drives being internal or encrypted, so it seems this is a different problem. Here are pictures of my interface for both.




Actually, my question looks similar to this one: osx split external hard drive partition

Is it the case that one can only repartition hard drives using the Mac OS format?
There's no such thing as "Built for Snow Leopard". That labeling just an old habit PC manufacturers have for Windows, but it doesn't apply to Macs at all. If Apple lets you run the OS, that means they tested it, and should work just fine. The reason you can't resize those disks is because they are formatted as "ExFAT", and OS X doesn't support resizing ExFAT formatted disks.