In the section of the manual on adding disks there is a minor typo as shown below WHOLE should in fact be WHILE.
Adding disks
Clear v Pre-Clear
Under Unraid a 'Clear disk is one that has been completely filled with zeroes and contains a special signature to say that it is in this state. This state is needed before a drive can be added to a parity-protected array without affecting parity. If Unraid is in the process of writing zeroes to all of a drive then this is referred to as a 'Clear' operation. This Clear operation can take place as a background operation while using the array, but the drive in question cannot be used to store data until the Clear operation has completed and the drive been formatted to the desired File System type.
A disk that is being added as a parity drive or one that is to be used to rebuild a failed drive does not need to be in a 'Clear' state as those processes overwrites every sector on the drive with new contents as part of carrying out the operation. In addition, if you are adding an additional data drive to an array that does not currently have a parity drive there is no requirement for the drive to be clear before adding it.
You will often see references in the forum or various wiki pages to 'Preclear'. This refers to getting the disk into a 'Clear' state before adding it to the array. The Preclear process requires the use of a third-party plugin. Prior to Unraid v6, this was highly desirable as the array was offline whole while Unraid carried out the 'Clear' operation. but Unraid v6 now carries out 'Clear' as a background process with the array operational while it is running so it is now completely optional. Many users still like to use the Preclear process as in addition to putting the disk into a clear state it also performs a level of 'stress test' on the drive which can be used as a confidence check on the health of the drive. The Preclear as a result takes much longer than Unraid's more simplistic 'clear' operation. Many users like to Preclear new disks as an initial confidence check and to reduce the chance of a drive suffering from ‘what is known as infant mortality’ where one of the most likely times for a drive to fail is when it is first used (presumably due to a manufacturing defect).
It is also important to note that after completing a 'Preclear' you must not carry out any operation that will write to the drive (e.g. format it) as this will destroy the 'Clear' state.
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