I play with OSs a bit, so a when I get my new MacBook, I obviously installed Windows via bootcamp. Later, Apple released something under NDA which also required a dedicated disk partition (for arguments sake, let’s call it Leopard), so I installed it over the top of my bootcamp partition.

Of course, one you reformat a bootcamp partition, Bootcamp assistent refuses to do anything with your system making it impossible to reinstall Windows. You can however fix this quite easily with the diskutil command line utility, without needing to repartition. Just reformat the drive as MS-DOS FAT32.

1. Run diskutil list disk0 to identify which partition number the bootcamp install is using (it was 3 in my case).
2. Reformat the partition with diskutil eraseVolume “MS-DOS FAT32” “NO NAME” disk0s3 (replacing the 3 with the partition number from step 1.

This will of course delete all the data on that partition, and as always you should make sure you have a full back up – but it will let Bootcamp recognize and start working with your drive again (including allowing you to restore your system back to a single partition).