Ken,
The drives will only hold a specific number of root directory files when used with the keyboard. I'm not sure how many they'll hold when used only with the PC. If you place things in folders, you can then fill those folders, and even place folders within folders and the keyboard's registration system can find them. The same is true with midi files.
Good Luck,
Gary
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Travlin' Easy
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)