Disk utility will not allow me to resize a partition on an unencrypted external hard drive
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.