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If you know that the memory reader always get the same drive-letter during install, just submit that drive-letter from your batch-file.

for %%i in (C: D: E: F: G: H: I: J: K: L: M: N: O: P: Q: R: S: T: U: V: W: X: Y: Z:) do if exist whateveryouwanttodo

For instance, if the drive-letter is "d:" , just remove "d:" from the above code.

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you could solve it simply by having them put in a card...

Another option is to use the "SetupSourcePath" registry entry. If I get time later I'll do a search, but there has been code posted on it.

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What if you use

SET CDROM=%~d0

That way you don't check all drive letters for the marker file. This command must be executed from a batch file on the CD. I mean it'll assign the %CDROM% variable to the drive it's launched from.