For many years now (10+) I've been in habit of keeping all personal & sharing files off the OS drive. And where possible installing the apps to a partition of their own. Thus having static & dynamic drives & partitions. So if a drive fails .. I don't necessarily lose important info. The harder working drives likely to fail before the others. And on OSX there are other advantages with VM & scratch disk options. Some apps behave better on a drive/partition of their own (on mac at least). But then, some OSX apps demand to be installed on the system drive & might refuse to run/update (properly) unless situated there.

Edit: I guess I should mention I have 4 internal HDD's. Two of these are partitioned. Due to the way OSX works, I leave the OSX system drive to itself so it can make use of VM without the drive needing to spend energy on anything other than the system where possible. I divide the disks & partitions up according to purpose (7). Video work, p2p & audio & video work, system, iTunes, applications, static documents, & an alternate system drive. As suggested above, I've used a similar work approach for many years on my previous mac computers. A handful of firewire 400/800 external drives for backup. Apple's Time Machine seems extremely good for backup.