I upgraded my rio 30g with an other 30g drive.
The rio's one is a Fujitsu. My new one is a Toshiba.
Since i got it, its impossible to get both working, only one at a time.
I switched master/slave jumpers and hd slots, but still the same... (it only worked twice in 3 days though).
Then I put the gun down, and did something stupid as a last try:
I took the drive jumper (fujitsu) and put is as 'cable select' (no jumper on the 2nd drive (!)).
And... it boots with both drives!!!

NB: Cable select is horizontal down as shown on the HD scheme, like this:
. .
==

So, if somebody has the same problem, or if sb can explain...

ya i had similar probablems with my dual drive recently.. i had it like so:

drive 0= master
drive 1= slave

worked fine for about a month, then all hell broke loose, it may or may not be a drive calbe problem.. mine looks fine.. but u really cant tell.. so then i broke down went everyone to try and find the right jumper pin.. and i did.. put it in same set up, still nothing.. jsut a puase at the boot screen.. so i went ahead and did a swap-a-roo on the jumper pins.. like so

drive 0=slave
drive 1=master

and for some gosh darn reason it booted right up.. so im waiting on the new drive cables from david.. and ill put it back the way i orginally had it and try that..

I got my new HD cable this morning, I also thought it was this @|#{@@ cable, but no.
I tried ALL the combinations with jumpers & HD position (it seams HDs don't react the same when they are on the 1st or 2nd cable slot):
HD1master-1st on the cable/HD2slave
HD1master-2st on the cable/HD2slave
HD1slave-1st on the cable/HD2master
HD1slave-2nd on the cable/HD2master
(I only had 1 jumper so I didn't check the dual cable select case).

But nothing was working... except this funny trick.

NB: both drives are bootable (2b11)

what are HD's costing you guys ????

Maybe putting the first drive 'cable select' is enough to select this mode, and we don't have to set the 2nd drive(?)
But if it was the case, it should be the same as setting the 1st as master and second as slave... but this way it doesn't work (!)

Mark Lord has mentioned that the proper IDE standard is to have Master in the 2nd slot (end of cable) and the slave in the first slot (middle connector).

Yeah, but when nothing mentioned works, we have to find an other way...