I'll add this: If the drive has been formatted using the GUID Partition Table (GPT) system, as most modern Mac drives are, then re-using it as a Master Boot Record (MBR) disk can result in a drive with contradictory MBR and GPT partition definitions, which can cause problems down the road. This is because the primary MBR partitions are all defined in the first sector of the disk. GPT uses a "protective MBR" in that location solely to keep MBR-only utilities from messing with the disk; the bulk of the GPT data occupy several sectors immediately after the MBR, with duplicates stored at the end of the disk. In an MBR disk, the sectors immediately following the MBR, as well as a few at the end of the disk, are usually unallocated. Thus, if you take a GPT disk and then repartition it with a GPT-unaware utility, the most important GPT data structures are likely to survive. This state of affairs could cause some utilities to become confused. In theory, you could end up wiping out your new MBR partitions if a disk utility uses the older GPT structures in preference to the MBR structures. (In theory, this shouldn't happen, but it might if the utility is poorly written or if you use it inappropriately.)

To avoid this problem, always completely destroy the GPT data structures. Apple's Disk Utility will do this when you tell it to create MBR partitions. GNU Parted will also do this when switching between label types with its mklabel command. My own GPT fdisk includes a couple of options that will do it -- one wipes out all GPT data and the other converts from GPT to MBR. I don't know what the various Windows partitioning tools do, offhand. Older tools, including Linux's fdisk and DOS's FDISK, are GPT-unaware and so can create a layout that's a ticking time bomb.