bug: USB disconnect during large file transfers via mpremote.

[nedseb]

Description

When copying large files (>1 KB) to the board's internal filesystem via mpremote cp, the USB connection intermittently drops mid-transfer. This makes it unreliable to deploy scripts or drivers to the board without using mpremote mount.

Symptoms

• mpremote cp fails with SerialException or OSError: [Errno 5] Input/output error
• The board disappears from /dev/ttyACM* and needs to be unplugged/replugged
• Not reproducible 100% of the time — suggests a race condition in the DAPLink firmware
• Small files (<500 bytes) copy reliably

Impact

• Cannot reliably deploy standalone scripts (e.g., calibration main.py) to run without USB
• Driver files (device.py) are often too large to copy reliably
• Workaround: use mpremote mount (requires USB connection during execution)

Workaround

Use mpremote mount lib/ to run scripts directly from the host instead of copying to the board's flash.

Root cause

Likely a race condition in the STM32F103 DAPLink USB CDC bridge. See steamicc/DAPLink#6 for the firmware-side investigation.

Related

• steami: USB disconnect during large file transfers via mpremote. DAPLink#6 — Firmware-side USB disconnect investigation
• steami: Stabilize config zone firmware (I2C IRQ race condition). DAPLink#5 — I2C IRQ race condition
• Memory: investigate RAM usage when loading multiple drivers #175 — RAM limitations (related: large scripts also stress memory)