Oh, right, I knew that.
When I first got the drive, I reformated with the Mac as HFS+. I quickly discovered that I could not mount under Linux, so I used fdisk to partition into two: a Linux and a DOS. Then I used mkfs to format both.
Some time later, I upgraded one of my Linux boxes from CentOS 4.0 to 4.4, then decided to try HFS+ again, so I once again reformatted with the Mac. Now it will mount under Linux, but read-only. So this time I think I'll just format the whole thing DOS.
So I guess my previous question was moot, as I was confusing partitioning and formatting. If I just use mkfs -t msdos to format, that should give me what I need. (Although, ideally, I'd rather have a 'modern' filesystem (HFS+, Reiser, ext3) that was read-write compatible on both the Mac and Linux.)