The most common BootMii problem is just not having the BootMii files on your SD card.
The not able to load a disk thing, though, it would be good to see your syscheck.
How long ago did these things work fine for you and is there anything you've done since then that might have changed that?
EDIT : And we'll just hope your drive isn't just broken. Also, is this for burned games or just retail games?