Steven,
What kind of data store is that? Is it a file or a database? You should not have problem backing up a databased because it usually is not locked completely. Regular files also can be saved even when opened if sharing attributes are specified correctly when file is opened.
We would also like to note that the problem of backing up application data should not be Cassini-specific. If you were able to solve this problem with IIS, for example, then you should have no problem with Cassini too.
Best regards,
UltiDev Team.
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