Awesome work!

Although, IIRC, removing magnet doesn't unmount SD card cleanly, just "cuts the power", So it may be good idea to prepare a pair of scripts for mounting/unmounting on demand. For people that use swap on microSD, it would also consist of moving swap to eMMC when card is to be put off, of course (and in reverse, after inserting it and willing to mount). I don't need to add that powering off sd card by means of magnet when you have swap there works as nicely, as physically pulling out RAM stick of your desktop PC while it's working.

/Estel