Trying to create my first RAID0 in a server.

I installed mdadm fine and all and proceeded to run:
mdadm --create --verbose /dev/md0 --level=stripe --raid-devices=2 /dev/sda /dev/sdb

With sda being my primary hard drive, and sdb being the secondary.

I get this error message upon running the command

"mdadm: chunk size defaults to 64K
mdadm: Cannot open /dev/sda: Device or resource busy
mdadm: create aborted"

I don't know what's wrong!

mdstat says:
"Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty]
unused devices: <none>"

Any ideas?

Hi,
hmmm, are you booted on one of theses partitions ??
Cheers.