splitted usb transfer instructions

docs/device/usb/troubleshooting.md

@@ -0,0 +1,24 @@
+# Troubleshooting Transfer
+
+You can’t drag and drop more than one hex file at once onto your micro:bit. If
+you try to drag and drop a second hex file onto your micro:bit  before the first
+file has finished downloading, then the second file may fail in different ways.
+
+When the first program has been written to the micro:bit, the drive will
+disengage. If you drag and drop a second file at this point it may not find the
+drive and the second write will fail.
+
+The errors may look like this:
+
+**Windows**
+
+![](/static/mb/device/usb-windows-copy-file-error.jpg)
+
+**Mac**
+
+![](/static/mb/device/usb-osx-copy-file-error.png)
+
+Or it may appear that there are two hex files on your micro:bit so the micro:bit
+won’t be able to run multiple files. To rectify this, unplug your micro:bit and
+plug it in again. Make sure that your micro:bit  appears as `MICROBIT` and not
+`MAINTENANCE`.