Why not just an Iomega ZIP drive?
Because the question wasn't "What would make an ideal doorstop?"
Seriously, I used to use a ZIP drive at home until one day 6 years ago I went to Fry's to buy disks to back up my (probably) 20Gb hard drive and realized that it would cost more just to buy enough ZIP disks than it would to buy a CD burner. So I bought a CD burner instead and have used that for backups until I got a DVD burner.
Now, the CD burner part is irrelevant here. I would advocate the method already suggested of getting an external USB hard drive. Make it FAT32 format, as I believe OS X can read/write to that. If you do that MAKE SURE you know the max FAT32 filesystem OS X can support. I ran into this problem with RH EL3. The 2.4.21 kernel doesn't support FAT32 filesystems beyond ~130Gb. Well, it doesn't complain if you make them, but after you put about 130Gb worth of data on them, they get subtly corrupted.