4 disks are configured with RAID 10. What's the  maximum number of disk failure for this RAID Group ?

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You can lose 1 or you can lose 2.  It depends on which disks are lost.  I will try to explain here.

In a RAID 10 you have 2 disks in a mirror group and the other 2 are in a separate mirror group.  Together they are striped.  That is what a RAID 10 is.

But let's look at just a single mirror group.  In a single RAID 1 mirror group, you have 2 disks mirrored to each other. You can loose either one of these disks, but not both.

Now picture two disks mirrored in your left hand, and two disks mirrored in your right hand. This is what a RAID 10 would look like.  You can lose one disk in your left hand and one in your right hand at the same time and not lose any data.  If you were to lose two in your left, or two in your right, then you will have data loss.

You can also go to this site.  Click on the server icon to see an animation to help visualize the setup.

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50% of the failure scenarios will survive a single HDD failure.  50% a dual failure. If, and only if there are no unreadable blocks on the surviving drives.

Remember you have billions of blocks on each disk.   If you have a 2 disk RAID-1 and just one block is bad on disk #1, and disk#2 fails, you still have data loss.

There is never full protection of data loss in all failure scenarios unless you have a full backup.  The only guarantee is that 100% of disk drives fail, .. eventually.

So plan for the worst, hope for the best.
Gerald ConnollyCommented:
RAID10 is a stripe of RAID-1's so as dlethe said, you can survive a failure of 1 disk in every RAID-1

4 disks = 2 * RAID-1 = 2 Failures
6 disks = 3 * RAID-1 = 3 Failures
8 disks = 4 * RAID-1 = 4 Failures

NB dlethe's comment on bad blocks and that as with Death & Taxes you can depend on 100% of disks failing (eventually)
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