Why do some files get backed up even though open, although most do not?
I’ve seen that MS Office Access database files will back up even though open, but I was surprised to see that Quicken files also do. But Quickbooks files do NOT.
What makes a file backupable even when open?
I suspect that it is a function of the application and how it accesses the data. If it really is being treated as a database, not just a file, the application won’t have the file open. Typically a database is not directly accessed by the application. Instead, the application is sending queries and other commands to the database system, another piece of software. A real database is only touched by the database software, its own program.
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Isn’t the real question here why are you doing a backup with files open?
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Why wouldn’t you be doing that? My Mac does a TimeMachine backup every hour, while I am using the machine or while I am not using the machine. All sorts of files may be open at any time.
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For many applications the database may not reflect what you see on the screen because of how the app moves data from memory to disk. This risks getting an incomplete state in the back up. If the data base crashes, there are typically mechanisms to back out the committed parts of such multi-part transactions … unless it is crappy software, of course.
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