Do you know of any method of backing up an Access mdb file that is
reliable if the file is in use? I don't
Depends on what you mean by "reliable." A file copy is not reliable
because you have no idea if the result will be in a consistent state
internally.
But a copy using Jet commands will result in a copy that is
internally consistent up to the time that Jet got read locks on each
object being copied. That seems to me to be good enough. The
TransferDatabase method would be one such method, as would Lyle's
SysCmd 6.
Now, if you mean "an exact copy of all the data I saw in my MDB when
I had it open in Access at the time I made the copy" then, no, you
can't 100% reliably copy that file.
But I think only idiots would want that kind of copy (what do you
want a copy for, anyway, other than backup?).