You need to make sure NOTHING is accessing the card.. and I do mean nothing.

If even a terminal is open and you are just "cd"'d into the media card.. it's considered busy. Or if your terminal is open as "user" who is in /media/mmc2, and you sudo su - to "root", and you cd out of the directory as root, and try to umount it.. "user" is still in the directory even though the active terminal is not. So you must first exit "root", change directories in "user", then go back to root and umount.

Or if a media application is running and even is aware there might be, possibly, a music file on your media card.. it could also be busy.

Or if you are booted into the media card with a cloned OS.. it's obviously busy..

If you are absolutely sure NOTHING you care about is currently accessing that device.. you CAN force linux to do as you wish by issuing:

umount -l /media/mmc2

Which is a "lazy" unmount.. it doesn't check anything and *can* cause some filesystem corruptions if done in the middle of read/write operations and things of that nature. It's typically a bad, evil, command that should only be used.. well, never . But I still find use for it sometimes.