Maybe the disk does not support SMART drive checking?
Or maybe you have a disk that is getting ready to fail.
Does it only happen when you have both disks attached but not when you try each one by themselves, or do you mean it does not happen when you simply unplug the secondary disk.
Did the problem just start? If not, did you do anything different?
Did you just add the second drive, or has it been working previously?
What motherboard, what exactly are the 2 drives? What do you have loaded on the drives?
When you guys post your problems, we love to try and help you, buy you HAVE to supply relevant information or anything we tell you is simply a crap shoot in the dark.