In November of last year we tested the Seagate Desktop SSHD 2TB. Now, Seagate also has a 4TB model of this 3.5'' hard drive with built-in SSD cache available. We have now also extensively tested this model.
For those who aren't yet familiar with the concept of an SSHD: manufacturers such as Seagate present it as "the best of both worlds", and we find ourselves agreeing with this to a certain degree. Essentially, an SSHD is a conventional hard drive combined with a smaller SSD which serves as a built-in cache. Data is first written to the SSD, and is copied to the conventional disk at a later point in time. At the same time, an SSHD attempts to place as much frequently accessed data as possible on the SSD portion, such that read speeds are also increased by as much as possible. This approach allows an SSHD to, to a certain degree, combine the low price per gigabyte of hard drives with the improved performance of SSDs.
The Seagate Desktop SSHD 4TB is clearly the big brother of the previously tested 2TB model. Again, we are dealing with a hard drive which consists of 1TB platters, four of them in this case. While the disks of the Desktop SSHD 1TB and 2TB ran at 7200 rpm, the disks in the 4TB model are limited to 5900 rpm. As such, we are basically dealing with an SSHD variant of the Seagate Desktop HDD.15 4TB.
The SSD cache of the device is, once again, 8 GB in size. Just like the 2TB model, the PCB features a single Toshiba MLC flash module, which internally consists of two dies. Seagate controls one part of the chip in MLC mode (2 bits per cell), and another part in SLC mode (1 bit per cell), the latter of which allows for lower latencies and higher endurance. The exact MLC/SLC ratio and whether or not a constant ratio is used is unclear: Seagate is not particularly forthcoming with technical details to the press. For those who are at all concerned about the flash chip's lifespan: Seagate indicates that it is twice as long as what would be required for five years of "standard usage", although they did not specify what standard usage exactly entails.
Just like the 2TB model, the 4TB model's SSD cache features a maximum read speed of 190 MB/s. The average read speed of the magnetic disks is a little bit lower than for the 2TB model at 146 MB/s, which can be explained by the lower rotational speed. Again, we must conclude that the advantage of the SSD cache does not lie in the increased transfer rates, but rather, in the greatly decreased access times.
Seagate uses an SSD controller from eASIC to control the flash memory. The regular HDD controller, which also features the Serial ATA 600 interface, is manufactured by LSI and features a 64 MB DRAM cache.
The 4TB drive has an average price of € 178 / £ 147 / $ 244, or 4.5 euro cents per GB. This is slightly more economic than the 2TB model, which costs 5.4 cents per GB. The difference in price between the 4TB SSHD and Seagate's standard Desktop HDD.15 4TB is, on average, € 28 / £ 23 / $ 38. This regular 4TB drive costs 3.8 cents per GB.