Today I installed FreeBSD 8 on a disk which was installed with OpenBSD 4.5.
This should be no problem you would think, however, FreeBSD and sysinstall seem to be greatly confused by OpenBSD disklabels. fdisk didn't see
any partitions, and my /dev/ had ad4a, ad4b, ad4c, etc.
Using fdisk and the disk label editor went fine, but when it tried to commit the changes it complained about missing /dev/ad4* entries.
The solution?
Burn a FreeBSD livefs CD, swap it for the install CD, and clear the first few MB of the disk:
# dd if=/dev/zero of=/dev/ad4 bs=16M count=1
What have we learned:
FreeBSD and OpenBSD disklabels are NOT compatible to the point it may actually break stuff.
Hope this saves someone some confusion and time