It's not necessarily bad to configure your RAID 5 array with *N* TB. What you need to worry about is how much data has to be read AFTER you lose a drive. Depending on the amount and size of the disks in your RAID 5 array, is what calculates your probability of failure for rebuilding the array.

What is a URE?

A URE (Unrecoverable Read Error) is when reading the sector of a drive fails, and it cannot be fixed. A URE number simply means that *"on average 1 error is detected while reading n bits"*. In our scenario, it's simply running a risk that it will fail to be able to read one of the distributed parities while rebuilding the array.

How to find out my probability of failure?

A typical consumer grade hard drive has a bit error rate of 10¹⁴. To calculate the probability, we would use the formula `Probability = 1-((X-1)/X)^R`

. *X* is the number of outcomes, and *R* is the number of trials. In this case *X* will be our bit error rate for the drive, 10¹⁴. *R* will be the size of the RAID array after losing a disk. For this case that is 8 TB.

Math Time!

First, we want to convert 8 TB to bits. This equals 6.4*10¹³. Now we can finally calculate the probability of our RAID 5 failing to rebuild after losing a disk.

Probability = 1-((10¹⁴-1)/10¹⁴)^(6.4*10¹³) ... plug this bad boy into wolfram alpha and you get 0.4727... multiply that by 100 and you have a 47% chance of your RAID 5 array failing to rebuilding in a 3x4TB setup that loses a drive. If you have a 4x4TB setup and you lose one drive, you have a 61% of the disks failing to rebuild.

So should I risk it?

The overall conclusion is that if you are using consumer grade hard drives, it can be risky to use large drives with RAID 5. It's a much different story with enterprise grade drives and hardware. As an example, using a Seagate Enterprise drive with a bit error rate of 10¹⁵, recovering a 4x4TB RAID 5 array that loses a drive, has only a 9% chance of failing to rebuild. The consumer grade hard drives has a 61% chance of failure.

In response to a comment by Christoper, I found this sleek RAID rebuild failure chance calculator. It quickly and easily calculates the probability of a URE failing during a RAID 5 or RAID 6 rebuild.

RAID rebuild failure chance calculator - created and maintained by magJ