A minimum of 2 HDDs is required to create a RAID 0.
While creating the RAID system, a "block size" must be defined. Using this "block size" the data is divided according to the block size and written alternately on each HDD.
For example, the first 64ko of the file are written on the first HDD, the next 64ko are written on the second HDD and so on.
This system allows to write or read parts of data on each drive at the same time. So the reading and writing speed is increased on a RAID 0.
Nevertheless, if a single HDD fails, the whole RAID 0 will be down. Indeed, the files are divided and reparted on all the HDDs, so all the HDDs are needed to get all the parts of a file.
The common RAID 0 failures are :