Did you check how the LEDs are actually wired in the drives? I would expect that the + pin is wired directly to +5V, though it may have the current limiting resistor in series with it, then the - pin will pull to ground to turn the LED on, low side switching is more common than high side. If this case you only want to connect one of the + pins.
You're right!
I double checked and found that indeed, the LED + pin goes through a 220 Ohm resistor and then on to the +5V pin of the harddrive Molex connector.
The LED - pin apparently goes through some circuitry to generate the signal.
I'm having trouble locating the LED pins of the other drive and suspect I've been tricked again by a current limiting resistor which would bypass a continuity test. I'll keep checking and see if one of the the voltage pins are common between the drives. In that case I'll be able to use a dual-coloured LED with a common cathode, or common anode (depending on the common voltage pin), right?
I do like the idea of using optocouplers as someone else suggested, that's a much safer and more flexible approach.
Yes, I don't fully understand it except it physically separates circuits. More complex than connecting a dual/multi-coloured LED, but something I'll look into more detail if that's not possible.