mdadm: Cannot open /dev/hde1: Device or resource busyI got similar output trying to add hde3 back to /dev/md3. Those partitions are only used for raid so it was really bothering me that it said they were in use. I googled around a bit and found a reference to the device mapper (which starts up on bootup for me because I use LVM) creating some devices based on a motherboard raid controller. You can get a listing of what the device mapper has created using dmsetup ls. I ran the command and sure enough there were 4 devices listed starting with nvidia_. I was able to remove 3 of the 4 using dmsetup -C
adj - subject to whims, crankiness, or ill temper
I use Gentoo Linux both at home and at work. Every so often I hit some snag and I'd like to detail the fix here both for my benefit and to possibly help anyone else having a similar issue.