Backup
Another issue has commonly been the speed of the backup. Media such as digital tapes can store a lot of data, but accessing them is relatively slow. Tapes can only be read or written in sequence, while media such as hard disks or optical drives are basically random access. When data is backed up, its encoding is often changed. This makes it possible to use codes such as Cyclic redundancy checks, which can detect, and sometimes repair an error.