1
Diagnostic step 1
Use a powered USB hub that has its own external power supply and test keyboard behavior. This is the simplest reversible test to confirm power as root cause.
FULL AI ANALYSIS
USB-C bus-powered hub causes external MIDI keyboard key repeats and dropouts on MacBook
Generated Oct 3, 2026, 12:35:03 AM
Analysis summary
Bus-powered hubs may not supply stable power under bursts or may have hub internal hubs/controllers that increase latency or drop USB transactions under load. MIDI devices are sensitive to latency and packet loss; duplicated messages and 'device not responding' logs point to the hub intermittently failing to service the device. The issue worsening on battery suggests host-side power management also participates. Because the hub also showed poor behavior on another host, the hub hardware or firmware is the most likely root cause.
Possible causes
Hub does not provide stable +5V power for sustained USB device operation (bus power limits), causing device resets or retries.
85%Hub internal controller firmware bug or poor USB transaction scheduling causing dropped IN endpoints.
65%Host USB controller enters a low-power state (USB selective suspend) and the hub/keyboard do not handle wake reliably.
40%USB cable quality or port handshake causing intermittent USB low/full speed negotiation failures.
25%Suggested fixes
2
Diagnostic step 2
Remove other devices from the hub and leave only the keyboard; test with MacBook connected to mains power (disable MacBook battery-savings mode) to see whether symptom persists. If stable on mains with single device, power budget on bus-powered hub is implicated.
3
Diagnostic step 3
Check System Information → USB when keyboard is connected through the hub and compare the reported speed and current draw vs direct connection. If the hub reports the keyboard as 'Low Speed' or shows power warnings, that's indicative.
4
Diagnostic step 4
Temporarily disable macOS power management features that affect USB (use caution; document changes), or boot into safe mode to test whether background processes interfere. Re-enable settings after test.
5
Diagnostic step 5
If hub shows same fault across multiple hosts and a powered hub resolves the issue, replace the bus-powered hub or file an RMA with the vendor. Escalate to vendor support if hub firmware update is available and you're unable to update it safely.