Internal hard drive issue with Mac Pro

Question

I work for two office that share information through a VPN. Each office has one Mac Pro being used as a server for data storage. The Pros have four internal hard drives (1TB each).
Hard drives in the Pro for OFFICE 1 were configured like so:
HD1 - OS/Boot drive
HD2 - Time Machine backup of HD1/DATA1/DATA2
DATA1 - Data storage
DATA2 - Data storage
Three weeks ago: Time Machine could not find HD2 and could not backup. I rebooted the machine and HD2 reappeared. However, after the reboot, DATA1 did not mount. I was unable to find DATA1 in Disk Utility. However, the drive could be accessed by all other Macs on the network. This machine was taken to the local Apple Store and we were told that HD2 was a bad drive and need to be replaced. We opted not to replaced the drive at this time and turned Time Machine backups off. We haven't had any problems with DATA1 since. (NOTE: During the weeks prior, the client machines intermittently lost connection with DATA1 and would have to reconnect)
Last week: OFFICE 2 - Mac Pro hard drives configured the same as OFFICE 1; however, HD2 was not used for TM backups. TM was turned on and set to backup HD1/DATA1/DATA2 to HD2 (exactly like OFFICE 1). The next day, TM could not find HD2 and backups stopped running. I rebooted the machine, HD2 reappeared, but we did not restart the TM backups.
Today: OFFICE 2 client machines have experienced intermittent disconnecting of DATA1.
I wasn't extremely concerned with the situation at OFFICE 1 as these Pros are almost five years old and in constant use. We plan on purchasing an external drive for the TM backups. HOWEVER, with a similar situation now being experienced in OFFICE 2, I am perplexed.
Could both machine be experiencing hard drive failures weeks apart? They were both purchased and installed at the same time.
Is there a known issue with TM and internal drives?
One difference between the two machines: OFFICE 1 is running OS X 10.8.5; OFFICE 2 runs 10.6.8
Any suggestions/help is appreciated. Thanks!

Answer 1

Relying on TimeMachine has some issues, along with only having one, I use at least two swapping TimeMachine so that they rotate AND I clone every drive (volume actually) so I can swap drives if need be, especially useful for t he system but good for any drive.
Power issues in t he buildings?
There are some programs that will scan for and report and./or attempt to reassign a failing sector ("fail" is qualitative, is 10 write failures a fail? then the 9th is not but it is a "weak sector.") Some run in the background.
Mirror can be 2 or 3 ( I prefer 3, that way if one fails I STILL have a mirror array in place) where any failure is not acceptable, AND where the array data is backed up usually - less important  when using a three-drive configurations.
How old, when last initialized (zero), and is it showing spare blocks being used? that last is the most indicative of failed drive.
Rare but a bad SATA cable or power (seeing how the cables are tucked away) is only way generally for multiple failures in same drive bay or computer. Note: early on in 2006-7 there were reports of some customers having motherboard and trouble with a drive bay , and t he problems showed up early.
Often though it is NVRAM or SMC related issue trigged by something else, a USB cable or device that is faulty, even a PCIe controller that is the culprit. And replacing cable, keyboard, device, and resetting SMC and NVRAM (often NVRAM needed to be done multiple times, but never keep resetting SMC) - unplugged and when you connect back up, do not install anything other than system, keyboard and mouse and see if that helps.
With a drive, likely the drive in 85% or more, is fine, but the directory is not, or it needs to be erased to rebuild all the system partitions.
1TB is small for TimeMachine. 350% of the amount of data, and always have free space for a full backup set of all the data. T.M. will try and prefers to automatically backup any and everything plugged in and connected, rather than my preference which is ASK FIRST to add a drive to TM backup set, otherwise leave it out. 600GB of data and system would mean 2.5TB drive, maybe 2TB minimum.