Comment 15 for bug 990744

mkswap fails because you appear to have something that looks like an MBR ( like a previous fat filesystem ) in that partition and mkswap has a foolish check where it refuses to wipe out an MBR that is triggered by this and the fact that you are using fakeraid. You can work around this by using dd to copy /dev/zero to the first sector of the partition. The proper fix for this is for mkswap to be invoked with a -f switch to force it to erase the MBR.