Assuming that the strategy is to use two external drives as mirror,
independent backups, then if the failure rate for a single drive is
50% in some time interval,
then you can expect a failure of one of the drives 75% of the time in
that same interval but only 25% of the time would you see a double
failure ( assuming that you just sat there after the first drive failed. )
In that same interval, the single larger drive ( using separate
folders to do a 'double' backup ) would fail 50% of the time.
In the first case, the data loss would be total only 25% of the time
with two drives and if you are alert and replace a failed drive
immediately, 0% or the time.
In the second case, the data loss would be total 50% of the time.
Of course, one needs to calculate the effects of having the primary
drive in the equation so the numbers would be:
For 2 external drives - 12.5% failure rate
For 1 large external drive - 25% failure rate.
Using two external drives sequentially so that only one is on-line
for backup at at time means that a catastrophic failure that kills
both internal and external drive at the same time gives a very high
level of protection.
Using only one external drive give significantly less protection and
allows for a single catastrophic failure to do in all of the data - a
real bummer.
So, What to do? Absent an external RAID setup, having two or more
sequential external drives is safest. Having only one external drive
is safe but still not catastrophe proof.
All that doesn't address the question of an external catastrophe like
a flood or tornado that gets the local system and all on-site backups
of whatever nature.
That can of worms is an entirely different discussion.
Cheers,
James
At 04:47 PM 4/27/2008 -0500, you wrote:
w8imo@xxxxxxxx wrote:
Even though hard drive technology and reliability have greatly
improved I personally don't like really big, like 1TB, hard drives
because if you lose it you've lost a lot. I prefer smaller drives
even if it means two or three.
But using two drives doubles the chance of a failure!
A single big drive has pretty much always been more reliable than
multiple smaller drives.
--
David Dyer-Bennet, dd-b@xxxxxxxx; http://dd-b.net/
Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/
Photos: http://dd-b.net/photography/gallery/
Dragaera: http://dragaera.info
James Schenken
