I usually write the eXtensions column on a Sunday. Before starting this week's offering, I restarted my MacBook Pro. Again. The replacement computer is on order, but as I want a larger hard disk than standard, there is a waiting period. Ordering directly from Apple's online shop, takes a week, but I prefer to support local businesses, who have invested into building the market here over the last few years. I am prepared for the delay.

As I anticipated a couple of weeks ago, there is speculation now about a new processor for the MacBook Pro range, but as I suggested then, my hard disk is not willing to wait for rumours to become concrete; and I always take the view that the moment I buy any computer, it is out of date. That may be more evident this time round.

A couple of points I made two weeks ago when I wrote about this replacement, caught the eye of readers and I had email concerning backups. As I am writing these words, the mirror of my website, which I keep on my MacBook Pro, is being copied to my iMac upstairs. For this operation, I use the Finder and login as a specific user (me) on the target machine, then I have access to the files and transfer them over my home network.

When I go to my office tomorrow, the first thing I do is to connect a 500G LaCie hard disk on my desk to the Firewire port (FW800) of the Mac. Time Machine then backs up the hard disk and continues this throughout the day. It does not back up the complete contents, which would take too long (and also take up too much space).

Using part of OS X that tracks file and system changes -- FSEvents -- the backups are incremental: hourly, daily and weekly. Hourly updates are kept for the current day and then consolidated into a day's updates. Using the interface, I can go back to the start of backups and retrieve any file, even if it has been deleted since. As an experiment last year in a class I erased OS X, reinstalled the OS, and then recovered the previous installation using Time Machine. I also have a hard disk that is configured as a bootable rescue disk with essential software, including Disk Warrior.

Initially, Time Machine only used Firewire, but now also uses USB connections. A version of the backup system is Time Capsule which incorporates the AirPort wifi router and a large hard disk of 1 or 2 Terabytes.

On another hard disk I have, I copy the contents of a directory to make sure I have recent versions of the files. Among the folders I archive are the entire iTunes folder which includes music and iPhone apps, my website mirror, teaching materials, and mailboxes (which are found in the Users Library folder). The entire mailbox folder is copied intact: removing any components may cause problems. On a new machine, I can use the "Import Mailboxes" command and will quickly be back up to speed. On that hard disk I also copy the Pictures folder which contains images from both Aperture and iPhoto. When my previous computer was stolen, I lost over 1,700 photographs, most of which I could not replace: that was a hard lesson.

To back up these folders I use software that came with one of my LaCie hard disks called Silverkeeper. It was created by LaCie, and is also available as a free download from VersionTracker. It is now at version 2.0.2 which is compatible with Snow Leopard.

I found it a bit slow to set up as I had to select source folders on the Mac and target locations on the hard disk, but once done, selecting each source manually finds the target and the contents are synchronised: new files are added and deleted files removed. While I prefer the manual approach, there are automatic settings, so the process takes place at scheduled times, and an option for backup on login.

One of the questions I was asked concerning backups was the ability to create a complete copy of the hard disk that is bootable. The only utility I know of that does this is Mike Bombich's Carbon Copy Cloner, now at version 3.3. I do not use this but user comments are favourable and it can be downloaded from VersionTracker or from the developer's site.

As well as a hard disk, many of us use other media. Most common is the flash drive, which may also have OS X installed for rescue purposes and the disk (CD or DVD). I also use SD cards. The latest Macs have readers for these built in, and on my current Mac I use a reader for backup, file transfer and for importing images from cameras. The MobileMe iDisk and synchronisation also mean that selected files and settings are available over several computers.