The choice of mass memory is the most thorny matter, especially in the field of content creation machines, like the PC for photo editing: in computer performance chain, it is the weakest link, so it’s important to find the fastest and most reliable solution.
In my opinion the best configuration consists in setting an SSD (Solid State Drive) as a system drive in which install the Operating System and every other software, storing the whole personal files and works (like photo catalog and other “static” files) into classic mechanical drives.
Going deeper in details, using an SSD for Operating System and programs makes the whole system extremely reactive because of the very low latency timings and high read/write speed: apart from the sequential r/w speed (which can reach up to 550Mb/s, that is 4 times faster than a traditional HDD drive), the most notable differencies can be found in 4K cluster management and access time latency to them. In this area an SSD can be orders of magnitude faster than a mechanical hard drive, a dramatic improvement.
But be careful in choosing the right SSD: a lot of them make use of controllers not 100% reliable, such as not so convenient and long-lasting NAND modules (such as the TLC NAND). I think that the best and most reliable units in consumer market so far are the Samsung 840 Pro, the OCZ Vector, the Crucial M4 and the Plextor M5 Pro Extreme, to say only a few. Their price is quite high (about 20% more than the others) but performance and data safety in particular are crucial factors in this case.
Talking about mechanical hard drive, to store static and voluminous data, I can’t refrain from suggesting the use of reliable units (not the “green” or “value” ones) in RAID1 or RAID5 mode. Losing years’ work is an incalculable damage so, relying on cheap drives without redundancy is the same as playing at Russian roulette.
Remember to check if the motherboard supports the SATA 6Gbit standard with RAID modes. Moreover keep in mind that in a RAID setup a certain amount of disk space is reserved for parity. For example: RAID1 with two hard drives of 1Tb each, makes available only 1Tb because all data written on a drive are simultaneously cloned on the other one. RAID5 with three drives (the minimum required) of 1Tb each, makes available the 66% of the whole disks capacity, that is 2Tb.
There are also differencies in speed: the RAID1 mode is as fast as single drive, while the RAID5 mode is nearly double compared to a single drive.
The most important feature of these RAID modes is the high reliability: in both cases, if a drive fails the whole user data is still safe. Replacing the broken drive with an identical one, the array will be automatically restored and resynchronised.
I think that the most reliable units in consumer market are the Western Digital RE (RAID Edition) or Velociraptor (for higher performance). In both cases the outlay will be quite high compared to cheap “green/value” drives, but reliability and performance will be on a much higher level.
About data safety: it’s best practice to do a backup of all your data on an external drive (such as an USB hard disk or a NAS unit) to connect to computer only in this case. Having a copy of all your work is an important insurance.