Do you use a single hard-drive or multiple?

I use a three drive setup usually:

• OS Drive - Windows, Drivers, Utilities, Swap, Temp - Small SSD
• App Drive - All programs and games - Large SSD or hybrid or High RPM HD
• Storage Drive - Documents, Downloads, Media - Large Hard Drives

In my actual gaming system, I have 2 app drives, one for steam, and one for everything else.

2 Drive System - With Network Mapping
SSD - OS, critical apps (web, office) and hardcore games (ex: eve online)
1tb - Storage, Games (steam) and all downloads land here

Mapped Network - Long Term Storage, Archives & Backups

edit: I pushed my swap to my 1Tb drive. While never proven I feel the read/write of swap beats on the SSD too much..while it would be faster on the SSD I worry about degrading the drive. (I have had 2x SSD failure to dying zones in the past)

I have a 4 way set up:
Computer:
1. SSD for OS and important stuff like some games and important apps
2. HHD 1TB for anything else
3. HDD 1 TB Steam Libary (installes every game xD)

Network:
WD MYCLOUD 3TB for Documents.

If you want the most reliable storage then buy a WD Raid NAS and then put in there WD Enterprise HDD... They are protected against movements so that they will not affect each other. But you can just research those particular specs. I personally just really like WD. One of my HDD is now running for 3 years (up time).

IMHO - Everyone needs network based storage. Either in the form of another machine (dedicated is best), a NAS (Network Attached Storage) or a USB attached to a lesser machine.