Symptom: When using the more recent version of the rather common SD card to IDE adapters in an A1200 or A600 (A4000?), the HD LED is always lit/active. Apart from this, these adapters works perfectly.

Cause: The motherboard should turn on the HD LED when the IDE /ACTIVE input is low. However, the transistor on the motherboard controlling the HD LED will switch it on whenever it sees more than ~0.6V difference from 5V on /ACTIVE. As the more recent version of these SD card adapter outputs 3.3V when there is no activity, the LED will never be switched of.

The SD adapter should reasonably have had an open collector/open drain output for the /ACTIVE signal both to be 100% compatible and to be able to coexist with another device on the IDE bus and this is what this fix will do.

1. Source a 2N7000 N-channel MOSFET
2. Cut trace going to IDE pin 39 (/ACTIVE)
3. Connect D pin to IDE pin 39
4. Connect G pin to 3.3V (for example on the common 3.3V side of the power/activity LEDs)
5. Connect S pin to the trace which was going to pin 39 before you cut it (for example on other side of the resistor connected to onboard activity LED).
6. Enjoy your Amiga, now with a properly working HD LED :).

See example.jpg for a visual example.