If you're trying to move data files on the same server to a different logical drive, then mutley1 is correct. Detach, move the file and re-attach the db.
If you're trying to move the database to another SQL Server completely, please describe how you're "moving" the db.
Are you backing it up and then trying to restore or are you trying to copy the data & log file and move them over to the new server? If the later, then it won't work. You can't copy data & log files while the database is in use and attached to the server.
Well, you can, but all you're liable to get is an error message like the one you have or a corrupted database.
If neither of these posts answers your question, please post the details of your move process so we can help you better.
Catadmin - MCDBA, MCSA
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