Time Machine backup .sparsebundle file inaccessible on new computer
Have a look here and try doing what he does (I know the problem is different).



The key came from a comment buried down near the bottom of the page. Running fsck kept getting clobbered with a "Disk In Use" error, but this solved it:

FWIW, I found that even with -noautofsck, snow leopard would auto-fsck, meaning I would get ‘resource busy’ or whatever. I have no idea why it is ignoring my no-auto-fsck request. Whatever.To allow the process to go, I ‘ps auxwww | grep fsck’ to find the PID, then ‘sudo kill (PID)’ to kill this fsck so I can run my own.

Killing the auto-fsck allowed me to run it myself. Not sure how long it took, I started it at 2am and it was done by the time I got up 6 hours later, but it was on a 500+ GB .sparsebundle, so I'm assuming it took a while.