I have an R320 with an H310 Mini RAID controller with a RAID1 array that consists of two identical disks.
One of the disks was marked as failed, I have rebooted and it is now marked as 'foreign'.
Is the combo of R320 and H310 controller hot-swappable? Can I replace the faulty HDD while powered so that I can rebuild the array?
The only question I would have is "does the server have a backplane?" If so, you'll be fine to do a hotswap. Most systems with PERCs do have backplanes, but not necessarily all. Easiest way to tell if you have a backplane is if the drives can be accessed externally. If yes, then you've got everything you need.
Do NOT power down to replace the disks if they are hot-swappable (accessible from the front of the server) … swap it "hot".