Db1.mdb

Db1.mdb (Access 2003) and Database.mdb (Access 2010) are files generated by MS Access when it either closes irregularly (e.g. crash, ctrl-alt-del), or often when it closes on a network file share.

See also https://social.msdn.microsoft.com/Forums/office/en-US/591921a3-475c-4980-98b1-b581dc2e29fb/bug-a-file-renamed-to-databasemdb-access-2010?forum=accessdev