Last year Western Digital launched the Red series of hard disks intended for use in NAS devices. Seagate also has such a series now called the Seagate NAS HDDs. We tested the 4TB model and compared it to Western Digital's disks.
Special NAS hard drives are more than a marketing gimmick. They have different software features and go through more rigid certification and compatibility tests. Seagate basically does the same with its NAS disks as Western Digital does for its Red disks.
Seagate's special firmware, called NASworks, features TLER, which stands for time-limited error recovery. What this means is that when a disk is unable to read a certain piece of data it will give up quicker, and after seven seconds will tell the controller there's a read error.
This is important for a NAS, because if a disk gets stuck attempting to read data and therefore becomes unresponsive, it will be removed from the RAID array. With TLER, the NAS can retrieve the necessary data from another disks and alert the user that there's a problem with the disk.
A second difference between normal disks and NAS disks is that they're supposed to generate less vibration when more than one are used in the same chassis. Seagate says it's a hardware-based solution provided by further balancing the motor inside the disk. This decreases noise levels and improves the lifespan of the disk.
NAS disks are also configured differently, in terms of power management in the firmware. According to Seagate the firmware is optimised for energy efficiency but also to wake up from sleep mode as quickly as possible. You can read the full review on Hardware.info.