For dual-DIMM configuration, you must use a pair of identical DDR4 modules.
The system is shipped without HDD. For the HDD compatibility list, please visit https://www.qnap.com/compatibility/
Card dimensions for PCIe slot:190 x 68.9x 18.76 mm / 7.48 x 2.71 x 0.74 inches.
Tested with drives fully populated.