Diode (for you non electrical engineer people like me). A diode is a electric switch that only allows current to go one way.
The 'hack' would take a diode (I suggest digikey [online store] or radio shack), which has one input and two outputs pins, which would 'split' the positive wire coming from the hard drive LED into two wires that connect to the motherboard and the Promise card. The negative wires could be split without the help of the diode.
The end setup would look something like:
Code:
(Motherboard pins)
(-)-------------------------
(+)---- |
\ |
(diode)---+(led)- -|
(card) / |
(+)---- |
(-)------------------------|
This would allow the card of motherboard to send a positive current through the diode, thereby lighting up the led (pretty cool, huh?).
The reason you need the diode is because without the positive current from the motherboard or card would attempt to go down through the other device (which would do nasty things to it).
This wouldn't be a very hard hardware hack (though I wouldn't suggest it for anyone who hasn't done some electrical wiring work before this). If you are interested, I can create a better diagram and a part list. It may be easier, however, to just buy another LED and drill a whole in the case for it (which gives you the advantage of having each hard drive light up independantly.)