There are several potential causes for that error. If the device is the Terastation Pro (TS-x.xTGL/R5), and the new drives are larger than 500 GB, the device will give that error since that model supports drives up to 500 GB. The Terastation Pro II (TS-Hx.xTGL/R5) is less limited, and supports drives up to 1 TB.
Make sure the drives have the SATA connector attached and snug. If you replaced the drives, you may need to update the firmware, since much of the operating system is located on the drives themselves.
