I have a BLE nano with a MK20. At some point in time, I did something weird and now I only see a "MAINTENANCE" drive on Windows and Linux. I have tried writing the bootloader.hex file to it but I get an error "In application programming aborted due to an out of bounds address." in the form of a file in the MAINTENANCE drive named FAIL.TXT.
I did have the unit working at some point, and I'm not sure how to get it to a factory configuration again. I'm unsure what I did to cause this, and I'm unsure if this is the nano or MK20.
Further, I see other files in there (DETAILS.TXT which provides details about the device and HELP_FAQ.HTM which points me to https://www.mbed.com/en/development/hardware/prototyping-production/daplink/daplink-on-kl26z/ which describes a similar situation in which I am, but copying the hex file provides the same error as above and does not get me out of maintenance mode).