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How to backup to hard disks (Mac)


title: How to backup to hard disks (Mac) created_at: 2012.02.13 updated_at: 2012.08.20 category: Resources platforms: Mac ---

Retrospect is able to make use of space available on external USB, FireWire, SCSI, or IDE hard drives for backup. Once the drive has been mounted on your local desktop it can be used as a backup destination. For information on backing up to a network drive, please see Technical Note No. 309 - Backing Up to File Servers with Retrospect.

To use an external hard drive for backup, you have several options: either back up to a file backup set stored on the hard drive or do a duplicate of your source hard drive to your backup hard drive. Retrospect 6.0 and later users can also use a "Removable Disk" backup set with USB and Firewire disks. This Tech Note is designed to outline the features and limitations of backing up to hard drives.

File Backup Sets

A file backup set is a single file that contains all the files you have backed up, and that can be stored on any random access device. Like tape or removable disk backup sets, you can incrementally backup to file backup sets and optionally compress your data. The drawback to a file backup set stored on a hard drive is that hard drives are not removable media and cannot easily be stored off-site for safekeeping. USB or IEEE 1394 (FireWire) drives that are hot swappable offer greater flexibility in this. Incorporating more than one backup drive allows for true media rotation, increasing the security of your backup strategy. A file backup set stored on a Macintosh volume is limited in size according to the format of that volume:

  • Mac OS Extended: 1 TB

  • Mac OS Standard: 2 GB

Unlike other types of backup sets, a file backup set does not have a separate catalog file upon creation. Its catalog is stored internally in the file itself, in its resource fork. (Its data fork contains the backed-up files.) Because a file’s resource fork is limited to 16 MB, the file will eventually grow too large for the file system. When this happens, Retrospect 5.0 or later will separate the catalog file from the data file to allow the file backup set to expand. The split catalog file will be named "Backup Set Name.cat." You can manually separate the catalog from the data at: Configure > Backup Sets > Properties > Options. The catalog file needs to be kept in the same folder as the data portion of the backup set.

Removable Disk Backup Sets

Retrospect 6.0 added an option under Special>Preferences>Media Handling called "Use USB/Firewire disks as Removable Disks". This option allows you to span between multiple USB/Firewire hard drives.

To use this type of Backup Set, check the "Use USB/Firewire disks as Removable Disks" preference. Create a Removable Disk backup set (saving the catalog file to the documents folder or Retrospect folder, not your external hard drive).


Retrospect offers another option for copying data to a hard disk: the duplicate feature.

The first duplicate operation will copy all files from the source volume, keeping them in Finder format. Subsequent duplicate operations will be incremental, copying and replacing files that have been modified or are new. Identical files are not copied again.

What is the difference between Backup and Duplicate?

  • Backup copies files in a proprietary format only accessible using Retrospect. Duplicate copies files in standard file format so they can be opened or used right on the backup disk without having to go through Retrospect.

  • Backups offer optional compression, not available with Duplicates.

  • Backups offer optional encryption, not available with Duplicates.

  • Backups can save old data incrementally so files deleted from the source are still available in the backup. Duplicate basically keeps a mirror image of the source so each duplicate operation overwrites previous data and only retains the current files.

  • Duplicates are always a one-to-one operation; one volume is duplicated to one volume. If you have multiple volumes to duplicate you will have to create an empty folder on the destination for each disk you wish to copy. You can then define those empty folders as "Subvolumes" from within Retrospect. This will allow you to copy Source volume #1 to Destination subvolume #1 and Source volume #2 into destination subvolume #2. The Retrospect User’s Guide contains detailed instructions on how to configure a folder as a Subvolume.


  • -34 (Volume Full) – The destination volume has little or no available storage space. You must free up disk space on the destination before attempting the next backup or duplicate operation.

  • Error -25051 (file backup set catalog limited to 16 MB) – Retrospect cannot save a snapshot or session because the file backup set’s catalog is too big. Upgrade to Retrospect 5.0 or later. You may also run a Recycle or New Media backup.

  • Can’t add that much data to backup set, limited to 2.0G

Last Update: August 20, 2012