Thanks for the suggestion, @felmue, but I figured out the problem!

The default baud rate of 460800 is too fast for this device. If I specify --baud 115200 (in platformio.ini, that would be upload_speed = 115200) it works.

Maybe this will be useful to someone else in the future.

Thanks for the suggestion, @felmue, but I figured out the problem!

The default baud rate of 460800 is too fast for this device. If I specify --baud 115200 (in platformio.ini, that would be upload_speed = 115200) it works.

Maybe this will be useful to someone else in the future.

Also, I updated the CP210x Macintosh OS VCP Driver to v6 and that didn't help.

I'm having trouble with uploading to an M5Atom using esptool.py on macOS Big Sur 11.2.2 with the error:

A fatal error occurred: Timed out waiting for packet header

I've tried to use the Updater_FW20200422 (suggested in another thread), but get another error with that tool:

Detecting M5Stack products.
Find M5Stack products.
Kernel driver deteched.
Apply magic success.

Fail to cast magic stage 2, please contact vendor.
Waiting for device in DFU mode.....................%

Otherwise the serial port works and I am able to use esptool.py to read_mac:

dean@lucky13 razzle % esptool.py --port  /dev/cu.usbserial-7D52471593 read_mac
esptool.py v3.0
Serial port /dev/cu.usbserial-7D52471593
Connecting....
Detecting chip type... ESP32
Chip is ESP32-PICO-D4 (revision 1)
Features: WiFi, BT, Dual Core, 240MHz, Embedded Flash, VRef calibration in efuse, Coding Scheme None
Crystal is 40MHz
MAC: 50:02:91:92:3e:e8
Uploading stub...
Running stub...
Stub running...
MAC: 50:02:91:92:3e:e8
Hard resetting via RTS pin...

Any suggestions on how to proceed?