This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
The circuit described below works well. It shows Red for Drive Enabled + Write and Green for Drive Enabled + not Write. That is, a Read is assumed if the drive is ON and no Write is occurring. The advantage of this approach is that you always have a lighted LED 'drive ON' indicator and, so, there is no need to drill a new hole in your drive panel. The new LED can use the hole occupied by the old "in Use" indicator LED. Parts IC- 74121 or 74LS121 1-Shot multi-vibrator Qgreen- 2N2222A gen purpose NPN transistor Qred- 2N2222A gen purpose NPN transistor Rcath- 120 Ohm 5% 1/4 watt resistor Rgreen- 3.3k 5% 1/8 watt resistor Rred- 3.3k 5% 1/8 watt resistor Rp- 20k 5% 1/8 watt resistor Cp- 10uF/10v 10% "dipped tantalum" capacitor LED- 2.2V 20ma 3-lead (common cathode) Green/Red bi-color LED Building the Circuit IC- locate pin-1 and mark it on bottom side with white-out. 'Dead-bug' mount the IC using epoxy in the open area near top-middle of board with pin-1 end pointing to the right. connect Rcath to solder pad at - (minus) end of C2 connect #20 wire from IC pin-7 to solder pat at - end of C2 connect Rp from IC pin-11 to IC pin-14 connect Cp + to IC pin-10 and Cp - to IC pin-11 connect #20 wire from IC pin-14 to solder pat at + end of C4 connect a wire from IC pin-3 to end of R16 closest to 74LS125 connect Rgreen to IC pin-1 (74121 "/Q" output) connect Rred to IC pin-6 (74121 "Q" output) solder transistor Qred Collector to Q1 power transistor Collector (solder pad area at left front of circuit board just to left of the power transistor). Position toward left edge of pad. solder transistor Qgreen Collector to Q1 power transistor Collector solder pad to the right of Qred. LED- connect a 3-wire 11" cable to the LED (black to center, red to to lead with right-angle bend, green to lead with slant bend). Old "in Use" LED- pop off the retainer ring and push through the LED. Leave the LED mount in the hole. Cut off the LED, spread cable ends and tab over with cellophane tape. (Put old LED and ring into parts box. Spread apart retainer 'leaves' of LED mount (at back side of panel) to permit easier insertion of new 3-lead LED. Run LED + cable over back of board and along bottom toward the hole in the front panel. Push through hole. (If hole is too small, use a Dremel tool and steel 'bulb bit' to slightly enlarge front part of hole.) connect LED cable black to free end of Rcath connect LED cable green to Emitter of Qgreen (right transistor) connect LED cable red to Emitter of Qred. position old LED cable beneath new LED cable. connect a wire from the free end or Rgreen (on IC pin-1) to the Base lead of transistor Qgreen. connect a wire from the free end or Rred (on IC pin-6) to the Base lead of transistor Qred. Add epoxy to the back of the LED and holder to secure the LED. How it Works When /Enable goes low to select the Drive, the power transistor (Q1) switches ON and supplies +12V at its Collector. If there is no Write, 74121 output /Q is high, Qgreen conducts, and the LED shows Green. That is: the Green LED will light during boots and for any READs. If the Drive is selected (/Enabled is low) and there is a Write operation and Write Protect is not ON, then, the /Write Request signal at 74LS125 pin 8 will go low. This triggers the 74121 One-Shot (at pin 3) producing an aprox. 160ms pulse at 74121 output Q. For the duration of the pulse, /Q is low and Q is high. Qred conducts, and the LED shows RED to indicate WRITE. By: Dan DeMaggio