external usb hd keeps disconnecting

[kohtaonic]

i recently bought 2 250gb SATA external hd's with phillips usb 2.0 housings which i have started to have some strange problems with.drive 2 works fine but drive 1 will disconnect itself randomly causing the "must disconnect otherwise damage to drive" warning to appear, then the drive remounts straight away. i swapped the cases on the drives and now suddenly drive 1 is fine and drive two disconnects, so i concluded that it is the case that is the problem.got new case and i still have the same problem.i have tried all 3 usb ports on my imac 17" 1.87 and tried different powerpoints for the drive but its always the same drive that disconnects, and if i swap them the problem changes from drive to drive.i thought it might be a power issue to my usb ports so i bought a self powered usb hub but it still happens. i have reset p-ram and am running 10.4.10 which supposedly fixed an issue with mounting/unmounting usb devices.
i am pretty stuck now so any help would be much appreciated.
thanks in advance.

 

[MisterMe]

Two possibilities:

1. Your may have improper connections within the case. Make sure that your drives' power and communications cable are securely seated.

2. Your enclosure's interal power supply may be defective rendering it unable to power two drives simultaneously.

 

[kohtaonic]

sorry just to clarify i have two hard drives and two enclosures both with their own power source. i thought the wiring must be defective too thats why i bought a 2nd enclosure, but the drive that always disconnects when both drives are connected is fine when used on any computer (i've tried all 3 of my imacs) on its own.this problem only seems to occur when they are both connected. this is why i bought a powered usb hub but it is still happening which leads me to believe its a problem with the operating system.
anyhow thanks for the response, i'll keep on messing around and see if anything changes.