What you see
The diagnostics transport state is “Actual disconnect”, or an active read ends with “The serial connection closed unexpectedly.”
Likely causes
- The cable or device was physically removed.
- Host power management, a hub, or unstable board power removed the USB device.
- A native USB reset caused the old port identity to disappear.
Diagnostic steps
Stop changing the hardware during an operation
Allow the failed operation to settle and preserve the editor source. Never change live wiring to diagnose an error.
Restore a stable USB path
Reconnect a known data cable and check recognition at the operating-system level. Avoid a suspect hub and review the board’s power arrangement.
Choose the current port
Use Connect board after the device is recognized again. A previous permission or USB identifier may not refer to the newly enumerated native USB port.
Verify and inspect
Verify before uploading, then open Serial Monitor at the sketch baud. If upload fails while the port stays valid, use the separate upload-failed guide.
Board-specific notes
Uno/Nano use their USB-to-serial bridge. Mega’s extra UARTs are not separate browser ports. Leonardo/Micro native USB may re-enumerate during reset. Pro Mini requires an external adapter with the correct logic voltage.
Safe recovery
Export the local sketch before further troubleshooting. Do not erase browser storage to recover a USB device; storage is unrelated to the physical connection.
Evidence and references
Reviewed 2026-10-23 against Circuitera’s actual behavior. An editorial review is separate from a physical board test.
Circuitera implementation evidence
src/connectionDiagnostics.js; src/serial.js: Stk500Transport.readByte