Hi, thanks for getting back to me.
I can say for a start that I also experience this problem when using a microsd card plugged directly into the unit.
I take your point re: the custom rom but it would seem to me that if it's possible to find and scan one track into the library, it should be possible for it to do them all.
Case in point, tracks that were previously in my library are now omitted following the full rescan. None of the filenames or locations have changed.
I can access and play all the files using the headunit's file manager, it only seems to be Poweramp that has difficulty in reading them.
The last time this issue occurred (on an older version) the only way I could get Poweramp to fully scan all my media was to scan the folders in chunks, rather than all at once. so A to D, E to H, etc, etc. The folders do contain approx 150GB of media, could it just be too much for the scanner?