There is no direct function or variable showing that a USB key is available or not and the related root path.  But you can use the following workaround:

depending on the status value you know
0  : USB key is mounted and the device has no partition table   --> You must use ´usb2msd:0/' for the root path of the USB memory stick
-1  : USB key is mounted but the device has a partition table      --> You must use ´usb2msd:1/' for the root path of the USB memory stick
-318 : USB key not mounted

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