AMITIAE - Tuesday 5 May 2015

Cassandra: Backup Redundancy for when Backup Disks Fail

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By Graham K. Rogers


I have a number of external disks that I use for Time Machine backups, for backups of data, and for rescue purposes: these have OS X installed. Some users never backup. Students and work colleagues laugh at me when I suggest this. Having had one computer stolen and disk failures on two Macs (one replaced under warranty) I am well aware that no one is exempt from outside forces.

The need for a backup was brought home to me dramatically when the 12" PowerBook I had at the time was stolen in a burglary. While I was able to retrieve many of the text files, I lost over 1700 photographs. Since then, I have used Time Machine on a regular basis, as well as backing up all the photographs (in several libraries) on separate disks. Disks, plural.

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.

This weekend however, there was a slight change. As I have a new Mac mini, I brought home all the disks while I have been setting the new machine up and have been alternating backups, until this morning. Time Machine preferences showed that the Imation disk had not completed its backup and I should check the disk if the problem persisted. I tried backing up again later, but the same happened. Time for a fix.

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.

Time Machine

With Disk Utility bowing out, I tried Disk Warrior. The repair was not wholly smooth and the report suggested (like Disk Utility) far more file and directory problems than should be expected. Nonetheless, the repair was completed - the directory was replaced - and I moved the disk back to the MacBook Pro.

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?

Time Machine

I will keep an eye on this. The moment that USB 3 disk shows any further problem, it will be trashed. I do have a spare USB 3 drive on my desk, but this was intended for data on the Mac mini. No matter. That is still being set up, so if I do need to use that disk, I have time to buy another locally. I also have the two other dedicated Time Machine disks.

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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