I just want to dissemble a raid0 and use one of this harddrives for other purposes, so I just stoped the mdadm with

/etc/init.d/mdadm stop

and I started to partition the disk /dev/sdc. After changeing the type of partition from Linux Raid Audtodetect to Linux I tried to format it with ext3 filesystem, but I got this error:

$ mkfs.ext3 /dev/sdc1
/dev/sdc1 is apparently in use by the system; will not make a filesystem here!

After some research I just the solution:

mdadm –stop /dev/md0

The linux raid utility is just marking this disk as used if you don’t really stop md0. It seems stoping the mdadm process doesn’t have nothing with mdam –stop command.