A RAID can be made with software in two different ways. In the case of Software RAID, the disks are connected like normal hard disks. It is the computer that makes the RAID work. This means that for each access the CPU also needs to do the calculations for the RAID. The calculations for RAID 0 or RAID 1 are simple. However, the calculations for RAID 5, RAID 6, or one of the combined RAID levels can be a lot of work. In a software RAID, automatically booting from an array that failed may be a difficult thing to do. Finally, the way RAID is done in software depends on the operating system used; it is generally not possible to re-build a Software RAID array with a different operating system. Operating systems usually use hard disk partitions rather than whole hard disks to make RAID arrays.