This command on a Raspberry Pi 3:
stty -F /dev/ttyACM0 ispeed 9600 ospeed 9600 raw
Resulted in this error:
stty: /dev/ttyACM0: Inappropriate ioctl for device
This happened after the sd-card (with the OS) of the Pi entered failsafe mode so it is read only. I used dd to copy the 16GB sd-card to a new 128GB one:
dd if=/dev/sdd of=/srv/iso/pi_20250928.isodd if=/srv/iso/pi_20250928.iso of=/dev/sdd
The solution was to stop the module, remove the device, and start it again:
# modprobe -r cdc_acm
# rm -rf /dev/ttyACM0
# modprobe cdc_acm
... as it was probably copied with dd from the read-only sd-card.
epilogue: The 16GB SD was ten to fifteen years old. The Pi expanded the new card to 118GB not 128GB as advertised: relevant XKCD https://xkcd.com/394/
