Since you mention that the machine is set up with RAID-1 (mirroring), then that will protect you against mechanical failure of the hard drives
Not to put too fine a point on it, but a mirrored RAID setup with 2 disks will protect you against mechanical failure of *one* drive.
Yeah, no kidding, and its very unlikely a computer is just going to, in the middle of the day, start screaming, "I lost a hard drive, I lost a hard drive!". A sysadmin (or someone with at least some basic expertise) needs to be actively monitoring the state of the server, reviewing event logs from time to time, etc. A RAID-1 (or even RAID-5) system will just soldier on, doing its work, not noticeably degraded in speed -- without 'telling' anyone, unless software has been installed and configured to provide for email notification.
Of course, any level of RAID does not protect against:
a) Viruses;
b) Malicious users;
c) Spyware;
d) Server admin messups;
e) Power surges or controller failure;
f) Operating system software bugs that can create filesystem corruption.
g) Drives being filled up, which can/often causes data loss when someone tries to save something to a directory that's full.
Seriously, if you're not a computer expert, this is not a place to DIY. A 5-year-old server, coupled with the modern tendency for people to use larger files, emails, etc., etc., may very well have some capacity issues.