hdiutil compact failed - internal error
Solution 1:
Update: I was running LenovoEMC's Storage Manager in another window and had a webpage opened to the ix2-dl drive.
After closing these applications, I did a full shutdown and restarted.
This time, when I tried hdiutil compact, it worked.
Just putting this answer here for anyone who runs into a similar issue. Most of the web pages that I found during searching were related to problems with sparsebundles that used FileVault.
Solution 2:
Same issue here, I had to mount/unmount the sparsebundle volume before running hdiutil to compact it.