How to disable software raid (mdadm)?

"dmraid -rE" did the trick. Removed the RAID array and everything is fine now.

Thank you everybody.

You can make drives forget they were in a RAID by zeroing out their md superblocks. Assuming your old RAID drives are known as /dev/sdc and /dev/sdd in your Ubuntu system, try the following commands:

sudo mdadm --zero-superblock /dev/sdc
sudo mdadm --zero-superblock /dev/sdd

If, for whatever reason, those don't work, then you could try more drastic measures, like zeroing out the entire drives with dd, e.g.:

sudo dd if=/dev/zero of=/dev/sdc
sudo dd if=/dev/zero of=/dev/sdd

but I wouldn't try that unless nothing else worked, since it will take a relatively long time, and it will delete all data on those drives.