Once image files are captured in the field, they have to be stored on a drive of some sort to be optimized and archived. Over time, collections tend to balloon beyond expectation. Thankfully, hard disk drive capacities have grown too, and there are 6TB and even 8TB (and larger) units available that should fill the needs of most photographers.
3.5” spinning platter HDDs have been around for a long time. Even though they have limited read/write performance and operate on a SATA 6Gbps bus, they are still the most cost-effective storage medium for consumers, and models running at 7200RPM with large caches perform reasonably well. Brands change over time, as do reliability reputations, but the king of the hill at the moment seems to be Western Digital’s 6TB Black. Keep in mind there are a whole array of “enterprise” HDDs too, like the recently announced WD Gold series, which are designed to operate in more demanding environments (datacenters, RAID arrays) and will likely have longer mean time between failure rates and longer warranties. They’ll also be more expensive, and the question will be whether it makes any practical difference in your personal setup.
There’s another category of HDD also, designed to draw less power (like the WD Green). These drives often spin at lower RPMs or have a variable spindle rate and thus will not be as “high performance” as other versions, but they can be useful as secondary drives or in external enclosures.
Keep in mind one of these large capacity drives holds a huge amount of data. Initial file transfer can take a very long time, so when you make the move, plan it out carefully. Also, the more data you put in one place, the higher the risk if you have a failure, so a rock-solid backup plan becomes even more crucial.
[Caveat: If you use a Mac Pro (2013), none of this applies with regard to internal storage as there are no internal HDD bays, though HDDs for external storage become an even more important choice as the flash-based internal storage is fast but rather limited in capacity.]