AMITIAE - Tuesday 5 May 2015
Cassandra: Backup Redundancy for when Backup Disks Fail
By Graham K. Rogers
Some may say that the three disks I use for Time Machine backups is overkill, but if one of the purposes of backing up is to be ready for a disk failure, what happens when one of the backup disks fails?
I run two Time Machine disks at home. One of these is an older Firewire 800 disk connected via a Firewire-Thunderbolt adapter. This churns away comparatively slowly, but does the job day after day. The Thunderbolt disk I have seems to be a little less reliable, although it is much faster. It has unmounted itself on a few occasions, and once I lost data.
No matter, I still have the Firewire disk backup and there is a USB 3 (Imation) disk at work. I have the luxury to be able to wipe a suspect disk and start backing up again.
Using Disk Utility on the MacBook Pro I started a repair of the disk, but this did not complete: perhaps because it was being used by Time Machine. I tried again, but the main partition reported a fault. Because I could, I moved the disk to the Mac mini but a repair using Disk Utility stopped after some 30 minutes. A long list of problems in red suggested to me that this might be more than a software problem.
The Time Machine backup process started correctly and was completed successfully, so for the time being, instead of a hardware problem, the disk and its directory may have become corrupt. The question is how?
Rather a redundant backup than no backup at all.
Graham K. Rogers teaches at the Faculty of Engineering, Mahidol University in Thailand where he is also Assistant Dean. He wrote in the Bangkok Post, Database supplement on IT subjects. For the last seven years of Database he wrote a column on Apple and Macs. He is now continuing that in the Bangkok Post supplement, Life.
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