When you buy a Western Digital (WD) 1 TB My Book External Hard Drive, they usually come formatted for Windows using FAT32. Of course, if you are using it with a Mac, then you’ll want to reformat it using Mac OS Extended (HFS+).
The problem: When you try to reformat in Disk Utility, you’ll get a very cryptic error that says that there was an unknown error with the partition map.
The solution: If you go to the partition map and click the “Options…” button, you’ll be able to select which scheme to use for the boot records. It turns out, if the boot record scheme is set to “Master Boot Record”, you will not be able to format it as Mac OS Extended. Flipping it to GUID or Apple Partition Map will work.
Once you get to that magic dialog, it’s pretty clear what to do. Why Apple doesn’t communicate this error intelligibly when you attempt to format I don’t know.
In any case, there is an Apple Discussion Board thread that saved the day for me. I’m posting it here just in case it saves the day for someone else.
I’m now ready to try out Apple Time Machine for the first time…
Update (12/29/2008): It’s amazing, but this continues to be one of the most popular posts on this blog. Everyone seems to have this problem. Interestingly, it’s not unique to Western Digital drives. I find that I need to do this with Maxtor and Seagate Free Agent drives also. I just did this with the new Seagate FreeAgent 1.5 TB external. Works like a charm…