Examples of data problems include the following:
Pool or file system space is missing
Transient I/O errors due to a bad disk or controller
On-disk data corruption due to cosmic rays
Driver bugs resulting in data being transferred to or from the wrong location
A user overwriting portions of the physical device by accident
In some cases, these errors are transient, such as a random I/O error while the controller is having problems. In other cases, the damage is permanent, such as on-disk corruption. Even still, whether the damage is permanent does not necessarily indicate that the error is likely to occur again. For example, if you accidentally overwrite part of a disk, no type of hardware failure has occurred, and the device does not need to be replaced. Identifying the exact problem with a device is not an easy task and is covered in more detail in a later section.