@Amol @Lucas This is why I always use QNAP. You guys rock.
Just to mention, I have two other (more modern) QNAP NAS in the rack, with this TS-1679U-RP being the only EOL unit.
To answer the drive question, I was considering getting a pair of Kingston Fury Renegade 4TB (SFYRD/4000G) drives for use with the QM2-2P-384A in my TS-1679U-RP. You may not need to test them, since they already appear on QNAP’s Compatibility List for the QM2-2P-384A:
Unfortunately, I could not find any deep technical spec sheets on these drives (Kingston is notably lacking compared to other manufacturers in this area), but here is what they have published: