iTunes on Windows OS resets media folder back to the default location on the C:\drive

There is a bug in recent builds of iTunes (well, since iTunes 12.9.4 I think) that, at least for some users, seems to stop the media folder preference from being saved properly. See Make a split library portable - Apple Community and put your library in a shape that isn't affected by the issue, and is easier to backup and move around in future.

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You need the library file stored, for example, at E:\iTunes\iTunes Library.itl. The default media folder then becomes E:\iTunes\iTunes Media. Should you inadvertently launch iTunes when the drive isn't connected you'll get a warning that the library cannot be found. Cancel, connect the drive, and launch iTunes once more. All will be good. With a split library iTunes would have silently reset the media folder in the same way that it (erroneously) keeps doing now.

tt2