RAID
Certain RAID levels allow to extend the array by simply adding hard disks, at a later time. Information such as parity blocks is often scattered on several disks. Adding a disk to the array means that a reorganisation becomes necessary. Such a reorganisation is like a re-build of the array, it can take a long time. When this is done, the additional space may not be available yet, because both the file system on the array, and the operating system need to be told about it. Some file systems do not support to be grown after they have been created. In such a case, all the data needs to be backed up, the array needs be re-created with the new layout, and the data needs to be restored onto it.