Thank you for your inquiry. You should check the following regarding the drives you are using. First, try formatting the drives outside of the unit. If you are unable at all to format the drives outside of the unit, it is a sign the drives may have an issue. Additionally, please make sure you follow these instructions on creating a RAID array on the MB662U3-2S R1.
First, put in the drives, connect the unit to the computer, and turn on the unit. Then, select the mode you want the disks to be on and hold the reset button in the rear for about 5-10 seconds. Once the front LEDs flash and you feel the drives do a short power off, release the reset button. Go to "disk utilities" and there should be a single drive, which should be the RAID array. Format that drive and the drives should be usable.
Let us know on the testing of the drive you are using and if you are able to create the RAID array on your unit.