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Old June 14th 18, 02:11 PM posted to alt.comp.os.windows-10,alt.windows7.general,alt.comp.os.windows-8
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Default External hard drive advice please

In article , Chris
wrote:

RAID is a bit complicated as the probability of a failure is highly
correlated. Firstly, the potential risk failure is if *any* of the disks
fail, which is the sum of the probabilities which for a 5 disk array is 5 x
0.1 = 0.5 (50%). Again not realistic.


it depends on the raid.

with raid 0, *any* failure loses the array. raid 0 is for speed, not
redundancy.

with raid 6, *two* drives can fail at the same time and no data is lost.

Probability increases with age of the disk, so a 5 year old drive is more
likely to fail than a brand new one. This is where correlated failures
occur, particularly with RAID5, as when a disk fails the array needs to be
rebuilt putting a large strain on the existing (likely old) disks which can
cause another one of them to fail. The array is now dead and unrecoverable
which is one reason why RAID5 is not recommended.


while that's true, the reason raid 5 is not recommended is due to the
chance of an unrecoverable bit error, which with the density of modern
hard drives, is statistically a near-guarantee to occur, and if that
happens during a rebuild, the array is lost.

In terms of raw disk failure probabilities RAID arrays are no more reliable
than separate disks, however, the redundancy and checksums allow for
seamless recovery from failures.


a raid is significantly more reliable for all sorts of reasons, other
than raid 0 which is used for speed, not reliability.

This is why a RAID is not a backup.


no, a raid is not a backup because it's a single device.

the advantage of a raid (other than raid 0) is uptime. if a drive
fails, the raid stays running and users can continue to access their
data.

a business cannot afford downtime while a new drive is obtained and
they restore from a backup. a home user probably can.
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