We recommend replacing a hard drive the minute you begin to see read or write errors from a particular drive. Unfortunately most people ignore the early warning signs.
RAID 6 allows up to 2 drive failures, however RAID 6 will use an additional ONE drive worth of capacity. If the system is 16 disks, it won't make a huge difference. Let's say your array has 8 disks, in RAID 6 you do take a capacity hit...leaving you with a total of 6 drives worth of capacity.
Have a backup plan, we repeat, have a backup plan.
If you have a backup plan, go with RAID 5 and buy a shelf spare (drive).
If you don't have a backup plan RAID 6 is a better option for you. There are advantages and benefits to both RAID levels.
The general rule is that your computer should be off before the device. If you do not intend on shutting down your computer simply unmount the drive from the operating system (MAC: Right click -> Eject. WINDOWS: Safely remove hardware)
When turning on your device be sure to boot up the device first, then turn on the computer.
This occurs whenever you use a RAID format that has redundancy, a portion of the total space is set apart so that it has the information to rebuild your RAID when it breaks down. It is normal and part of the RAID process.
The idea the lost of space is due to the way data is calculated differently on a computer which results in drive space ending in awkward numbers such as 931 GB for a 1TB drive. Instead of selling 931 Gigabyte drives they decided to just round up so things would be simpler.
A recoverable read error occurs when a bad sector is read but it was still able to read it. Generally speaking these will happen on occasion due to wear and tear on drive usage. You should only be concerned if there are multiple recoverable read errors on the drive over a long period of time. This usually means that the drive will start going bad soon so having a replacement on hand is ideal.
The main things you should be looking for when storing your device is to make sure the room is well ventilated and not excessively warm. You should also make sure to have ample room behind your device to have room for the device's fan.
The biggest cause of this issue is normally a bad drive in the RAID array. Check your log files or SMART indicators to see if there are any problems with your drive. When a drive goes bad it can cause rewrites to happen on its sector and depending on how bad the drive is, it can slow down computers to a crawl.