Disc drives ejecting on their own

Hello,

I have a 21.5" iMac and I have attached a 4 bay NAS device with two 3TB HDD's. When I power the device on my Mac see's them and then they disappear and I get a message saying they were disconnected incorrectly. They typically show up and disappear several times and then I get nothing. I go into disk utility to try and mount the drive again but this option is greyed out. The drives are formatted as exFAT so I could transfer the files from a PC. Anyone have a clue why these drives will not stay mounted?

Thanks!