How to remove an Apple Software RAID with OSX installed on it.
I decided to remove the Apple Software RAID, which consists of two 1TB HDDs, on my Mac Mini Server (Mid 2012). After booting the System via Internet Recovery ( CMD + SHIFT + R) and starting up Disk-Utility to remove the RAID-Volume, DU won't do that, because it was not able to unmount the Disk. Why?!
I checked with Terminal which process blocked and found, that notifyd was holding a filehandle on that volume (only the volume mountpoint). Killing notifyd caused a short system-hang follewed by 20 open filehandles on that Volume. Don't try to kill notifyd permanently (via "kill -ill" and some combination of launchctl commands - your system will freeze).
Finally I tried to remove all things from that volume ( cd /Volumes/OS && rm -rf * && cd /) and killing notifyd once more. The process re-spawnd and aquired a filehandle on the volume once more - but I gave Disk-Utility another try. It complained again that it was not able to unmount - but did it nevertheless.
Conclusion is, that removing a Apple Software RAID with OSX installed on it is quite not easy but possible.