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Inside Macintosh: Files /

Chapter 2 - File Manager / Identifying Files, Directories, and Volumes

Volume Reference Numbers

A volume reference number is a unique number assigned to a volume at the time it is mounted. Unlike the volume name (which the user can change at any time and hence may not be unique), the volume reference number is both unique and unchangeable by the user, and so is a reliable way to refer to a volume for as long as it is mounted.

Volume reference numbers are small negative integers. They are valid only until the volume is unmounted. For example, if you place a volume offline and then bring it back online, that volume retains the same volume reference number it was originally assigned. However, if you unmount a volume and then remount it at some later time, its volume reference number might not be the same during both mounts.

Note

A volume reference number refers to a volume only as long as the volume is mounted. To create a volume reference that remains valid across subsequent boots, use alias records. See the chapter "Alias Manager" in this book for details.