The majority of large capacity drives has been four terabytes for some time but Seagate is the first company to go further by recently introducing a new six terabyte version of their Desktop HDD.15 lineup. One of the big differences between this and their 4TB version is the doubling of the cache to 128MB to handle the larger block sizes for the drive. The extra high capacity does carry with it a higher price tag of around $300. In fact, the price is significant enough that it is almost twice the cost of the 4TB version of the Desktop HDD.15 from Seagate. Now the spindle speed for both these drives is a bit slower at 5900rpm which means that the performance does suffer a bit compared to a 7200rpm drive at the same capacities.
The best value for a hard drive is one that gives you the best storage for the price. This is typically calculated by the price divided by the storage to yield a price per gigabyte. When using this, the best overall choice for value is the Toshiba PH3300U-1I72DT01ACA 3TB drive. Priced consistently around $130, it comes in with a very low $.043/GB. Performance is reasonable for the drive especially considering its price. Perfect for adding some additional storage or using as a backup drive. If you are looking to get a bit more capacity, then the Seagate Desktop HDD.15 4TB at $160 is a good option but you really need a 64-bit version of Windows and appropriate hardware to fully use the drive.
Performance is typically determined by the overall spin rate of the drive and the data density. The controllers on the drive can also make a difference particularly if the drive is going to be used for certain tasks such as databases that require a lot of non-sequential data access. For tasks like this, the lower capacity but faster spinning Western Digital VelociRaptor 1TB drive offers the best overall performance. In fact, the drive is really a 2.5-inch drive designed for enterprise use that comes in a special 3.5-inch drive tray to help with cooling. Pricing ranges from $200 to $250. If you need capacity as well as performance, than the Western Digital Black 4TB offers a significantly larger capacity with a similar $250 price.
RAID is a configuration that uses multiple drives to enhance performance, data reliability or both. Because the drives work in tandem and rely on having similar specs to ensure the best overall performance in the array, the drives require a bit more. Those looking to put together a RAID array would be well served by looking at Western Digital's SE drives. These drives are specially designed to be put into RAID and network attached storage arrays. The 4TB versions is the largest capacity model available and offers solid performance plus a five-year warranty. Pricing is a bit high at $280. Since most consumer RAID uses don't necessarily require this level of performance, just about any other drive in this list in a pair will work well and probably be more affordable. If you need a more affordable model, the 2TB version is just $140.
Hybrid hard drives are not new but they try to bridge the gap between the performance of a solid state drive and the capacity of a traditional hard drive. Seagate has been in the business of hybrid drives the longest and are the most readily available. Their Desktop SSHD 2TB offer a very large capacity combined with an 8GB solid state cache to boost performance. Now, they aren't as fast as a solid state drive but it can help with booting and loading frequently used programs especially with non-Windows based systems. Priced around $120.
While the majority of the drives on this list have been for capacities greater than two terabytes, there are many older systems out there on the market that are either running the 32-bit version of Windows or using older technology where they cannot properly use drives larger than two terabytes, because of this, you want to purchase a drive of two terabytes or smaller. For just such a case, I highly recommend the Western Digital Black 2TB drive. It uses the same technology as the other Black series drives that offers it some great performance just with smaller capacities. It spins at 7200rpm with 64MB of cache and comes with a nice five year warranty. Priced at $145 it can be a bit expensive compared to the higher capacity drives. If you need something a bit more affordable, the Seagate Barracuda 2TB is a very reasonably $90 but just a two year warranty.