Well, I rarely use my laptop for DAW purposes, but I occasionally do. Its drive died. Brand was Seagate. In the same week, I also had two Seagate desktop drives malfunction fatally (one mechanical, one electronic) in my MythTV setup.
Seagate is now on my do-not-buy list.
I've also had occasional failures of IBM (mostly difficulty spinning up after sitting for a while or bearing noise problems w/o data loss, but I had an 80 GB Deskstar eat itself once). I lost a Maxtor once. I've lost a dozen Quantum drives back when they were still in business.
All hard drives suck. My rule is this: if it is important, keep at least three copies on separate drives. Keep at least one drive off site.
BTW, your question about whether you have a separate system drive or not is largely unimportant. What kills hard drives are three basic things:
1. Temperature outside the safe range.
2. Design flaws.
3. Mechanical impacts (can be cumulative damage to head arms and similar in the case of laptops, too).
Assuming you cool your drive correctly and don't drop your computer, a desktop drive should last forever in the absence of #2. Unfortunately, #2 isn't rare anymore. Within the category of design flaws, there are a few frequent categories:
1. Head failures due to improperly designed parking ramps. This causes damage when the heads are parked (spin down the drive) repeatedly.
2. Bearing lockups/leakage. These issues are exacerbated by heat, and are most commonly seen when a drive first spins up.
3. Faulty air filters. This causes dust to get inside and wreck things.
4. Stupid electronics failures. This includes bad caps, spin chip failures, overwritten track 0 sync bytes due to a buggy drive firmware, etc. These really suck, and there's not much you can do about them.
In short, the best thing you can do to maximize your drive's life is to disable energy conservation and leave it spinning 24x7. Oh, and don't cool the drive too much. The recommended range is 36-47C (97-117 deg. F). According to Google's stats, drives that were running at 27C (87 deg. F) or cooler had twice as high a failure rate as drives within the optimum range.