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General considerations

Backup copies must be kept physically separate from the disk containing the original data. You should make your backup copy either across the network onto a disk on another computer, or onto removable media.

Backup across the network

You need to remember the limitations of central file space. It can't accommodate copies of all the documents on everyone's hard disk. With this proviso, you might consider backing up to:

  • Your central file space. This is highly secure, in so far as the home directories are themselves backed up by IT Services, but space is strictly limited. IT Services cannot provide a general service enabling everyone to copy all their documents from their desktops and laptops into their home directories.
  • Shared/group central space. This too is highly secure because the centrally managed servers are backed up by IT Services, but group file space is provided for shared access to data and restricted, password-protected access to confidential material, and is not to be used for personal documents and other files that should kept in your own personal file space.
  • A locally managed server in your own department. In general this option is not recommended, and departments should consult IT Services before setting up their own servers.
  • Backup to another user's PC using Windows file-sharing, sometimes referred to as peer-to-peer file-sharing to distinguish it from the above cases where you backup your data to a server. This option is not recommended on security grounds.

Backup to removable media

The following types of currently available removable media may be considered:

  • Optical storage: of the different current standard options (CD-R, CD-RW and miscellaneous DVD) we recommend using CD-R (CD-Recordable) on grounds of reliability. That is: compatibility (maximum likelihood of being read on another machine), longevity, and non-erasability - you can't accidentally erase data written on a CD-R (and you can in fact regain old versions of data from a multi-session disk). An '80-minute' CD will take something over 700 Megabytes of data. The process of burning a CD is simpler now than in the past.
  • External hard disk drive: These can equal or exceed your internal hard drive in capacity, and very portable options mean that it is easy to keep the backup disk in a separate secure location.
  • USB memory stick: these are extremely portable and easy to use for short-term storage and portability, but their long-term reliability has been questioned; even quite new memory sticks have been known to fail. Use of USB memory sticks for backup is not recommended.
  • Magnetic tape: these are used for very large-scale data storage such as the centrally managed backups. They are less easy to manage than the other media referred to, and recovery of individual files can take very long time. Tapes vary in capacity but will typically hold tens of Gigabytes.

Backup generations

You have to restore a file from backup when you find that the current version on your hard disk is damaged or deleted. Occasionally the restored copy itself turns out to be damaged. This happens when the backup copy was made after the damage occurred but before it was detected.

To guard against this sort of frustration you should keep more than one generation of backup. The centrally managed backup tapes are kept for six weeks before being over-written.

When you make a backup copy containing the latest version of a document, you should not over-write the previous backup of the document, just in case your latest version has introduced some sort of corruption into the file, or you have made some inadvertent changes to the contents.

To avoid overwriting, you should either have a set of removable disks and use them in order, or you should rename the files or folders to contain the date on which the backup was made.

The exact procedure to be adopted depends upon what sort of backup you are carrying out. Although you must keep more than one generation of backup, there are limits to how far back you should go. These limits are partly practical limits on how much data you can accommodate, and partly a matter of how long you should, from the point of view of both law and policy, retain copies of your documents. Note the above comment about writing multi-session CD-Rs.

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