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That really doesn't sound like an ArcSDE geodatabase -- SQL-Server usually reserves the suffixes
on database files (and they come as a set .mdf/.ldf) -- but if it is, then you need to use SQL-Server
to detach the database, move it (and its companion) using the filesystem, then reattach the mdf.
The .sde file is more than likely just the connection file to the database, so you'll probably want to
look at the datafiles' names (and location) before you detach the database.
- V