Tested boards: pca10056, pca10059
Steps to reproduce the errors:

• take a blank board, connect it to a computers USB, put a simple main.py on it (LED blink, or such) and run it
• create a subdirectory on the virtual drive (e.g. /lib)
• copy some CircuitPython libs into the subdir (a few 10kB of files are usually enough, but sometimes more is needed)

The following errors occur, sometimes immediately, sometimes after disconnecting and reconnecting the board from the USB:

• script crashes and shows reading errors in main.py
• all files on the virtual drive become read only
• some files on the virtual drive become corrupted or deleted
• the whole drive is wiped and appears freshly formatted

This happens with all 4.x beta releases, as well with self-compiled versions. Increasing the stack size doesn't help.

Could anybody please try to reproduce this? Maybe also on other boards?