I have 2 HDD's connect every so offten it will start beeping... WHY?

this happens every few days, I leave it beeping and it will quit beeping.
It just happened and the green light on HDD 2 of the NVR is flashing..
ANY HELP PLEASE..
CAN OR IS THERE SOME PLACE THAT WILL EXPLAIN HOW THE RECORDING SYSTEM WORKS.... STORES... BEEPS FOR WHAT REASON OCATIONLLY...
THANKS JOHN