Time Machine

Ever since Apple introduced Time Machine in Mac OS 10.5 Leopard, I have been using it with all three of my Mac machines. In the Apple community, Time Machine seems to create a split between those who use it and come to rely on it (although often with other back up strategies as well) and those who find it doesn’t work for them or has other side effects.

Like many others my initial back up took a long time — as I recall it took about two weeks for my initial back up to complete to an attached Firewire drive on my MacBook back in January 2008. Part of the reason for the time taken was that the laptop is not permanently connected to the drive, but nevertheless it was an exceptionally long time even by comparison with a SuperDuper! full backup. However, once the initial back up was complete, the subsequent incremental backups in my case are generally quick and unnoticeable, even when my laptop has been travelling with me for 10 days and I return and plug in the Time Machine drive.

It does take some care to make sure that if you are doing something which creates lots of temporary files that these don’t get backed up every hour. If you are doing something like video editing you could fill your Time Machine very quickly. I exclude a number of directories for this reason.

Time Machine stores the backup and the increments such that unchanged files are not re-written, but links are created to them. On my iMac I have a 700GB drive which is barely half full holding Time Machine backups from December 2007. For my laptop which gets somewhat more use I had a partition of about 350GB which Time Machine eventually filled earlier this year after about 18 months of use. One of the consequences of the structure of the Time Machine drive is that all of the cross links cause Disk Utility to have a lot of work to do to verify a drive. I have accidentally unplugged Time Machine drives on my laptop and had power cuts on my iMac where for my own peace of mind I’ve chosen to do a Disk Utility verify disk afterwards. This takes a long time. On my iMac with 312 GB on my 700GB drive, the last time I did a verify disk it took over 70 minutes! Definitely an incentive to be careful when unplugging drives!

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