It means you must have two bootable volumes on your computer and be logged on to the second bootable volume in order to delete the file. It also means deleting the file
could easily would probably render the volume unbootable and it would have to be reinstalled, and the advice you received to leave it alone is sound. (Personally I would never attempt deleting such a file (unless I suddenly developed an irresistable compulsion to erase and reinstall the system from scratch.
).
AFTER THOUGHT: If you have a few hours to spare and and a coupe of spare drives, I have thought of safe way of deleting the file that
might work. But it will take several steps and each step will take some time to accomplish, and in the end might not work., but it would reduce the risk. If you are determined I can outline the process.
