Several issues with Feather M4 express (also simple blink aplication) on MacOS (10.12.6):
When using the mu editor it frequently hangs when trying to save code.py.
Sometimes code can be saved but is frequently modified to all 0xFF´s with exactly the same length than the original code.
Serial communication with Mu hangs, ctrl C crl D has no effect in this state.
Uodated Mu from 1.0 to 1.02, but doesn´t improve anything.
When using some other editor with the CIRCUITPTY volume code is correctly downloaded and executed directly after a reset of the Feather, but when saving it a second time the same problem appears. (volume hangs, code modified to FF´s.

Had similar behavior with beta 3 but seems that behavior has become worse