I'm not sure I fully understand why RAID10 is better than RAID5. Reading the RAID wikipedia article, they seem to imply that a typical RAID10 setup uses n*2 disks, where each block of data is written to two drives.

But how does that help in the case of drive failure? If a drive fails, then as size increases won't the exposure to a URE also increase? Is it better than RAID5?

When a drive in a RAID10 array dies, then the data on that drive is still directly available on a drive that's not protected by redundancy, and the other half of the data is still protected. Rebuilding the array requires reading a single drive without error. Rebuilding a RAID5 requires reading n-1 drives and computing parity without error.

Indeed. Reed Solomon coding (maybe only available in system software, not disk hardware) gives you efficiently balanced striping+redundancy

In a 4 disk system RAID6 can survive any 2 disks failing but RAID10 can only survive 2 disks failing if you get lucky. Once you go past 4 disks, RAID10 becomes quite expensive and yet can still be killed by a 2 disk failure event.

The window for losing two disks is reduced though, because rebuilding a single drive failure is much faster in a RAID 10 system where it's just a drive copy, as opposed to n drive reads and a parity calculation.

Work out the probabilities. The results may surprise you. They did me. I was like you once.

I work at a hosting company, and we're using RAID10 exclusively.