Backup Solutions - Copy files to disk

I'm looking for a simple backup solution which will copy selected directories on a PC to an external disk. Here's the catch - it must be able to handle drives being rotated. So for example if the external drive is mapped as F: backup to that, but when F: is taken offsite and when the H: drive is plugged in, use that since F: is gone. Rotating the F: and H: drive every couple of days.