I managed thousands of HP Prolient servers with various RAID setups in a large datacenter (64,000 sqft). There are usually 2 situations in which i'd replace drives, upon failure or upon predictive failure. However, the latter IMHO is much like "at some point in the future this drive is going to fail." I've heard it has something to do with comm or monitoring issues with HP.
I would suggest you always replace a disk if it has failed even if the server has a hot spare. I hate using this phrase but think murphy's law. Yet a few things come to mind.
- Level of support with HP. Do you have a contract for support with them?
- How critical is the box?
- Field tech accessibility. Can a CE get there quickly?
- Part availability from HP.
- A rather big issue is department budget. Can you afford to replace them?
Again, these are just a few things that come to mind.
As for preventative measures, a good clean environment with operating temps and humidity within ASHRAE (a quick blog post here) or manufacture specs is about the best you can do. Also, good monitoring goes a long way to catching these issues as they happen to minimize downtime. You could take a look at Hp Insight. Personally, i stay away from the hp agents and strictly monitor servers from their integrated lights out (iLo) interface with help from HP SIM and an in-house ticketing system.
As for workstations, the above can still apply. You should never be in a situation where you have to recover from backup. And simply replacing disks to replace them is no way to run a server/workstation. Then again, i don't manage workstations so some of this may be overkill.