Repair Time Machine sparsebundle that will no longer mount
I have a write up on how to try and repair NAS-based sparsebundle errors on my blog.  In summary:


hdiutil attach -nomount -noverify -noautofsck /Volumes/{name of your disk}/{name of}.sparsebundle

You will then see something like

/dev/diskx Apple_partition_scheme
/dev/diskxs1 Apple_partition_map
/dev/diskxs2 Apple_HFSX


Where x is the disk id for the external disk. x might be 2, 3, 4 or
higher. You are interested in the one labeled Apple_HFSX or Apple_HFS. 
fsck_hfs -drfy /dev/diskxs2 using whatever relevant device was located in Step 1.

Hopefully, you will eventually see


  The Volume was repaired successfully

hdiutil detach /dev/diskxs2




However, since OS X 10.6.3, Time Machine will refuse to write to a destination volume that fails its verification.  Even if the process above succeeds in recovering the backup, you may still need to remove the black marks that Time Machine wrote when it failed verification.


Unlock the sparsebundle

chflags -R nouchg /Volumes/{name of your disk}/{name of}.sparsebundle

Move it back to its original location

mv /Volumes/{name of your disk}/{name of}_YYYY-MM-DD.sparsebundle /Volumes/{name of your disk}/{name of}.sparsebundle

At the top level directory of the sparsebundle, edit the file com.apple.TimeMachine.MachineID.plist.


Remove

&lt;key&gt;RecoveryBackupDeclinedDate&lt;/key&gt;
&lt;date&gt;{whatever-the-date}&lt;/date&gt;

Change

&lt;key&gt;VerificationState&lt;/key&gt;
&lt;integer&gt;2&lt;/integer&gt;


to

&lt;key&gt;VerificationState&lt;/key&gt;
&lt;integer&gt;0&lt;/integer&gt;