An additional reason could be that you're pointing to a workspace on a drive that no longer exists, thinking that you're choosing the valid one. For instance, for me the workspace used to exist on the F drive, but now it is on my D drive. Even though I don't have the F drive anymore it is still listed as a workspace I once used during Eclipse startup. When I choose this old workspace Eclipse complains that the workspace is "in use", which is very strange.