Question : Problem: 2 HDD Failed. How to indentify the cause?

Hi experts,
A nightmare happened saturday night here as 2 of 3 hdd (raid 5) failed. Impossible to rebuilt and data recovery failed. Fortunatelly we had backups so we're up again, on a single hdd (still scary)...Before doing anything (buying hdd and or raid controller and or brand new server (getting old)) we would like to know why it happened. Can you help me with all the possibilities. Thank you (sorry for my bad english)

3 MAXTOR 18GB ATLAS 15K ULTRA320 SCSI 68PIN 3.5" LP
1  Adaptec SCSI RAID 2100S
1 disk was "optimal" the other 2 were "failed"
Array = Dead - multiple drives failure.

Answer : Problem: 2 HDD Failed. How to indentify the cause?

It's very hard to say what might have happened.  If these are 18GB HDs then I have to assume they are pretty old (i.e., at least 3-4 years old if not more).  It's one of the known weaknesses of RAID arrays that, because you have 3 (or more) identical hard drives, they are more prone to multiple failures due to age.  In other words, all the drives will tend to get to their end-of-life condition at around the same time.  So, that's a possibility.  Were there any other environmental factors that might have played a part?  These would be things like excessive heat or dust/dirt in the server room, electrical brownouts or surges on a server that's not properly protected from such things, repeated server crashes due to other software or hardware failures, etc.

It's very doubtful that the controller has anything to do with it - these controllers aren't that prone to failures.  Also, if you had any array diagnostics running you would have seen problems earlier if the controller was at fault.