You don’t say if they are USB2 or USB3 thumb drives.. I’ve had weird issues sometimes with USB3 and switching to a different thumb drive made everything work. Since they are cheap I would obtain yet another one, different brand, and test it. Also, you could try rebooting idrive by holding volume button in for 30 seconds.
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-Steve
2015 BMW X1 35i MSport // BMS Intake
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