iMac.sparsebundle” is already in use ...any idea what this is ?

See Here...

The Backup disk image ... sparsebundle ... is already in use.

From Here > http://pondini.org/TM/Troubleshooting.html

If you are running Mountain Lion and getting this message when trying to backup, try this:

Open AirPort Utility (If you are in Mountain Lion this will be 6.0 or Higher)

Select you Time Capsule

Select the Disk Tab
Deselect "Enable File Sharing"

Click OK Let the Time Capsule Re-Boot

Go Back into AirPort Utility and re-select "Enable File Sharing" and let it Re-Boot again.

This should fix the problem.

If you are not running Mountain Lion the advice provided by other should work (http://pondini.org/TM/C12.html). The main thing is to reset users.

Worked perfect. Thank you!

Awesome - issue resolved. 🙂

Wow! that worked, thank you kg-bear! But what is causing this so I can avoid it in the future?

But what is causing this so I can avoid it in the future?

There are bugs in Lion and Mountain Lion. Keep those "fixes" handy, as you will likely need them again soon.

Bob is correct it is a known bug in Lion and Mountain Lion. I usually get the problem when there is any update to the operating system or if I have a power failure/fluxuation during backup. Apparently the system sees your backup file as being in use by "another user" (you the last time you backed up) and will not let you access it. All the work arounds for this problem involve somehow resetting users. The process differs if you have Lion versus Mountain Lion. Glad it worked for you. This had been a major frustration for me as well.

Thanks kg-bear, worked perfectly

Agreed. Bear-a-riffic fix. Thanks.

Bearilliant! Thanks 😎

got it working - Thanks

Thank you! It worked!

Thank you so much.

KG - you the man! 🙂 New to Apple and was in pants land for a bit there. Good people like you make the difference

After five months (original posting) you ARE still da man! THANK YOU!