HDD spinning drives are great. Just have multiples for backup if you wish to never lose your data.
I keep hearing others say that they don't fail, and if they do, the data is easily retrieved. Hell no... From a bad experience I know that to be not true.
Backup to an external drive, then backup that drive to another backup drive. Redundancy is better than loss of data/music files...
Apologies if any of this is redundant; I read through the thread and didn't see anyone mention some of the following stuff but I could have missed something somewhere.
First, why what I say is relevant: I build all my own PCs and have built a few dozen systems of various sizes for many different kinds of studios, from project studios to major Chicago rooms (I built a massive 24-rack space, multi-processor server that runs FOUR separate rooms on 20 drives at once back in January...but those drives are solid-state...more on that below), and I've been building PCs since I was a six year old in 1984. (My dad is a geeky tech guy who happens to love music. Haha.)
First, as Jimmy points out, he's had a bad experience with traditional spinning hard disks (HDDs) failing, which they do all the time, and if you think they "never" fail, I've got some land in Florida to sell ya.
Just think logically: almost ANYTHING with moving parts is gonna go bad at some point without regular maintenance, and "maintaining" an HDD in the sense of which I speak (physically maintaining, not data maintenance) is nearly impossible without a lot of experience and a few special tools and such.
Also, there are multiple issues with not only tracking to external HDDs but also with real-time sample access from them as well (this depends heavily on how yer particular DAW accesses and uses samples; for example, Reason uses a far less CPU/drive intensive sample playback method than does ProTools). Industry standard HDD access speed for tracking audio is 12ms (preferably faster), and as the storage amount gets larger, that access time tends to get larger as well because of the nature of how spinning drives work. Now, if you are an absolute Nazi about defragging those HDDs, you *might* get away with tracking to them, depending on other factors within your system; bus speed and the compatibility between components thereof will greatly affect the access time to an external HDD. Personally, I would NEVER track to an external drive UNLESS it was a solid-state drive (SDD), which means there are no moving parts, and even then, all other factors would need be at top performance. (For example, if your external SDD has an access time of 5ms and a fast bus speed but your motherboard or USB board's bus speed is 5x slower than the SDD's, your data might not be able to transfer fast enough to keep up with the tracking speed requirements.)
I agree with everyone who suggested RAID setups; they are a no-brainer. HDDs are much cheaper per gig than SDDs, especially as the storage space increases, so using HDDs for a RAID setup makes perfect sense as long as *at least* the primary/master drive is internal. The associated slave drive can be external if one properly sets up the RAID system, but most people just put it inside the box, as there's typically no real need to take a RAID slave with you anywhere.
Personally, if I were you, I would invest in at least ONE good-sized SDD (256gb or bigger) and use that as your tracking drive. If you can afford it, I'd get two SDDs and use a smaller one (128gb or so) as yer system (C
drive and then the larger one for tracking and sample access; then I'd use HDD's to RAID both of those SDDs, and I'd still use an external to back up my tracking/sample drive *separate* from the RAID system. The only real issue I have with RAID systems is that since they're automatic (for the most part), anything that infects the master is automatically sent to the slave. If you're physically backing up just your tracking drive by dragging the session files to it the external, there's less of a chance of those files getting screwed. But that's only slightly less of a chance.
Also, if you do go with any HDDs larger than 1Tb, make sure you partition them into separate drives. This will not only extend the physical life of the drive and improve performance but also helps safeguard against the entire drive becoming corrupted or infected, as when a single drive is partitioned, the OS recognizes them as separate physical drives (for all intents & purposes). I personally would never use a traditional HDD larger than 1Tb. Just too much chance of corruption and failure overall, and a whole Tb is a TON of data to lose in general but *especially* to data corruption. I can't imagine losing two or even three effin' terabytes of recording data. I'm sick just thinking about it.
If you go with SDDs, be sure to read up on the differences between them and HDDs; they require different maintenance and have different operating requirements. For example, *never* let an SDD operate with less than 10% free space available; it drastically reduces the drive's performance, which isn't nearly as true with HDDs.
Hope that helps. Once again, sorry if any of this was redundant.