During a recent migration from Exchange 2007 to 2010 I found I was unable to remove the public folder store from the Exchange 2007 server.
It was returning the following error when using remove-publicfolderdatabase or using EMC on Exchange 2007.
Remove-PublicFolderDatabase : Object is read only because it was created by a future version of Exchange: 0.10 (184.108.40.206). Current supported version is 0.1 (8.0.535.0).
Obviously the Exchange 2010 server had touched the database in some way, probably due to the Offline Address Book migration.
The fix was quite simple - remove it using the Exchange 2010 Exchange Management Shell. Can't use the GUI as the Exchange 2007 public folders do not appear in there.
Get-PublicFolderDatabase -Server EXCH2007 | Remove-PublicFolderDatabase
Where "Exch2007" is the name of the Exchange 2007 server.
After removing the database I refreshed the GUI and was then able to drop the Storage Group and complete the removal of Exchange 2007.