I just spent that last couple of hours trying to figure out why I couldn’t create a new software RAID set on my Ubuntu 10.04 system. Long story short, it turned out to be device mapper grabbing hold of the drives at boot. No amount of lsof would show that the devices were busy. The key was running dmsetup table and seeing that the drives in question were indeed “locked” by the device mapper.

This thread was the key I needed to get it all figured out – http://www.mail-archive.com/[email protected]/msg10661.html

After issuing dmsetup remove followed by the device name shown in dmsetup table I was off and running.