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