I DO know perfectly well how to partition an external HD. On my Mac I have the original OS X partition, but I have also recently installed Windows in its own partition. Is it okay to partition my 1TB external HD that I use for Time Machine, and use one partition for backing up OS X with Time Machine (as TM won't let me back up Windows) and use the other partition to back up Windows? Thanks in advance
That is not a problem at all. Only issue I can maybe see is if the Windows backup software wants the second partition in NTFS format, Disk Util won't be able to do that for you. You would need to use Windows to format the NTFS partition.
Format into FAT32 in OS X and then boot in Windows to format into NTFS. Just one word of warning... how are you planning to "back up" Windows? Are you going to use Windows backup? I have tried and it backs up perfectly fine, but when it comes to restoring I ran into lots of issues... it restored perfectly fine and also deleted my OS X partition. I now use Winclone and it works perfectly fine. You won't need to repartition your external drive and the files Winclone creates will be backed up by time machine.