In this glossary, you'll find definitions for backup-related terms, which may also appear in the text of the book in blue. If you're reading in the electronic version of this book, note that you can click any blue-colored text to follow its link and then return to where you were with a menu command or keyboard shortcut, as noted in Navigating Back to Where You Were.

additive: When a backup copies files that are new, renamed, or modified since the last session without deleting or overwriting older versions, that backup (normally an archive) is additive.

additive incremental archive: A type of backup in which files that are new or modified since the last run are added to an archive, without replacing or deleting earlier versions of those files. Also called a versioned backup.

AFP: Apple Filing Protocol, the network file-sharing protocol used by Mac OS X's Personal File Sharing.

archive: An archive is a copy of your files as they appeared at multiple points in time, sometimes stored as a single, larger file. Some backup programs use the term archive to refer to a backup in which the original files are deleted from the source volume after being copied to the backup medium.

block: A unit of storage for files, whose size varies according to volume size and format; typically 4,096 bytes (4K) on modern Macs.

block-level updates: A way to transfer data in which a backup program incrementally updates an archive by copying only the portions of files that have changed, rather than the entire files. (For more info, see Block-level updating, p. 95.)

client: A program that works with a server program is a client. For instance, Retrospect Client is a small program you can install on each of your computers. Retrospect Client communicates with the full version of Retrospect on the server, which does the bulk of the work. The computer running client software is often called a client as well.

client-server: A type of network backup system in which client computers use a small background program to send files over a network without mounting a volume in the Finder. Backups are initiated by the server and stored on media connected to the server.

Combo drive: A Combo drive is an optical drive, standard on some Macintosh computers, that can read from DVD media and write to CD-Rs and CD-RW media.

data fork: Although this is less common in Mac OS X than in previous versions of the Mac OS, Macintosh files can be composed of two portions, a data fork and a resource fork. In general, the data fork holds data for the file—text, graphics, video, and so on—that could be relevant to any platform, whereas the resource fork stores information that's relevant only when the file is used on a Mac.

destination: The volume (hard disk, partition, optical disc, etc.) to which files are copied during a backup. Also called target. Compare with source.

differential: A type of backup in which each run copies all files which are new or modified since the initial full backup. For more information, see the sidebar Incremental or Differential? (p. 16). Compare with incremental.

disk image: A container for files and folders that can be manipulated (moved, copied, deleted, backed up) as a single file. When you double-click a disk image, the Finder mounts it as a volume.

duplicate: A duplicate is a complete, exact copy of your entire hard disk that (if it's stored on, or restored onto, a hard disk) you can use to start up your Mac if necessary. Also called a clone or a mirror. See The Duplicate (p. 12) for more details.

FTP: File Transfer Protocol, a common method of transferring files over the Internet.

hard link: A pointer to where a file or folder is stored on your disk. A given file or folder can have many hard links, each of which functions exactly like the original file. (To learn more, see the sidebar The Magic of Hard Links, p. 152.)

incremental: A type of backup in which only data that is new or changed since the last run is copied. For more information, see the sidebar Incremental or Differential? (p. 16). Compare with differential.

local: Think of local as meaning "part of your computer." If you save a file to your Mac's hard disk, you are saving it locally. In contrast, you can save it remotely on a file server, which could be down the hall or on the other side of the globe.

media spanning: The capability of a backup program to split data (possibly even a single, large file) across multiple optical discs or other media—and rejoin them when restoring the files.

metadata: Extra information about your files beyond their obvious content, such as modification date, Finder labels and comments, and access control lists (ACLs) used by file sharing. Resource forks are sometimes also considered metadata.

mirrored: A RAID configuration in which identical data is written simultaneously to two or more disks; also known as RAID 1. A mirrored RAID can sustain the loss of any member disk, because all data is stored redundantly.

multisession: The ability to record additional chunks of information on a partially used optical disc as separate volumes after the initial write session. Some applications, including Retrospect, can add data to partially-used optical discs using a packet-writing technique; this does not create additional volumes, and it means that only the program used to record the discs can read them later.

NAS: See network attached storage.

NDAS: See network direct attached storage.

network attached storage: Typically refers to one or more hard drives with their own Ethernet (or wireless) interfaces. Compare with storage area network.

network direct attached storage: A type of network attached storage that uses proprietary technology from Ximeta, which allows a drive attached via Ethernet to behave for the most part as if it were directly attached via FireWire or USB.

off-site: When backup media is kept off-site, it is moved to a separate building from the one where the original data is stored.

optical media: CDs (including CD-ROM, CD-R, and CD-RW) and DVDs (DVD-ROM, DVD-R, DVD+R, DVD-RW, and DVD+RW). So named because they rely on lasers to read and write data. For more info, see Optical Media (p. 66).

peer-to-peer: A type of network backup in which two or more computers back up to each other, without requiring one to function explicitly as a backup server.

pull: A backup initiated by a server, in which data is copied from a mounted network volume (a client computer) onto media connected locally to the server. Compare with push.

push: A backup initiated by a client, in which data is copied from a local disk to a mounted network volume. Compare with pull.

RAID: Redundant Array of Independent (or Inexpensive) Disks; a way of combining multiple physical hard drives into a single local volume. See striped and mirrored. I give more details in RAIDs (p. 62).

resource fork: Although this is less common in Mac OS X than in previous versions of the Mac OS, Macintosh files can be composed of two portions, a data fork and a resource fork. In general, the data fork holds data for the file—text, graphics, video, and so on—that could be relevant to any platform, whereas the resource fork stores information that's relevant only when the file is used on a Mac.

rotating archive: A backup scheme in which new or modified files are added to an archive incrementally (without overwriting recent versions), but files older than a certain date (or backed up more than a certain number of days ago) are removed to save space.

rotating backup: A backup scheme in which a complete copy of all selected files is made during each run, the newest set of files replacing the oldest of two or more previously copied sets.

SAN: See storage area network.

script: A set of instructions for a backup program to follow. Scripts may include source, destination, schedule, and other options.

server: A server is a program that sends information to client programs. Backup servers, for instance, work with backup clients to copy files from networked computers onto centrally located media. A computer running server software is also typically referred to as a server.

SMB: Server Message Block, the network file-sharing protocol used by Windows and Mac OS X's Windows Sharing. Sometimes referred to (slightly inaccurately) as Samba.

snapshot: A list of all the files in the designated folders at the time a backup runs. Backup software that uses snapshots generally enables the user to restore data to its state at the time of any backup with a single operation.

source: A source is a folder or volume from which data is copied during a backup; the data's original or primary location. Compare with destination.

storage area network: A device comprising one or more hard drives able to be shared among several computers, generally via high-speed FireWire, Fibre Channel, or SCSI connections (rather than an Ethernet-based network). Compare with network attached storage.

striped: A RAID configuration in which blocks of data are alternately written to two or more disks; also known as RAID 0. A striped array produces a volume with the combined size of its member disks, but does not have fault tolerance.

SuperDrive: An optical drive, standard on many Macs, that can write to and read from DVD-R media and CD-R or CD-RW media.

synchronization: The process of maintaining identical copies of a file, folder, or volume in two or more locations. (For more on this topic, refer to Synchronization Utilities, p. 14.)

verification: The process by which a backup program confirms that each copied file is identical to the original.

versioned backup: See additive incremental archive.