This system allows a secure storage, thanks to the RAID 1, but also a faster reading and writing, thanks to the RAID 0.
It's less reliable than a RAID 10, because if a HDD is down, it will cause the whole pair failure.
In a RAID 01, files are divided in blocks of X sectors. The first block is written on the first HDD of a pair, the second block is written on the second HDD of the same pair, and so on (a pair is a RAID 0). Then this pair is cloned on another pair (the two pairs are like a mirror, this is a RAID 1). This system offers reliability and good speed.
A RAID 01 is less secure than a RAID 10.
The common RAID 01 failures are :