Drives randomly dropping from RAID array

Solution 1:

It's the fanout cable arrangement...

I have drives like this that work in external HP enclosures and in HP servers where there are internal 3.5" bays and a proper backplane.

Does this mean that you're not using an HP server? What are you using? Where are the disks? How are they connected to power? Molex connectors direct to the drive? Molex power injectors on the SATA breakout cable? You're not using them on a backplane, so you're not taking advantage of hotswap, etc.

Finally, are you SURE you upgraded the firmware of the controller to the latest revision? You may have to do this from within the OS if this is not an HP server.