[AccessD] What is this .md1 That Is Locking My Security File?

Now Access is being rude to me.  I recently moved the security file that
controls my database from each person's own hard drive to a centralized
location on the network.  Every couple of days, the security file named
rb.mdw has a friend, rb.md1, locking out most of the users.  Does anyone
know what this is and how to prevent it?  Thanks.