Source HD fails during BkUp

[cslloyd]

If a backup has already been made on an external drive and a Smart Update is being performed and the source HD fails what happens to the backup? Is the file in the process of being copied deleted?

What is the best procedure to protect the backup in case of a source HD failure during backup?

[dnanian]

Since the file is copied in place, there a chance of losing that file, yes. There's really no wait to avoid this... there's always a window of vulnerability.

This kind of potential, rare occurrence is why we recommend rotating multiple backups for some users: please see the Introduction in the User's Guide for a full discussion.

[cslloyd]

So the file would actually be lost? If the file were a file that existed in the backup and was being updated then the file in the backup would be lost?

I guess the program erases the file in the backup first and then copies the updated file to the backup. Is this why it is lost? I was hoping maybe the updated file would be copied to the destination HD successfully and renamed to replace the previous version.

[dnanian]

You can't really do that without requiring substantial extra disk space needs -- imagine, for example, copying a 2+GB Entourage image or huge modified movie file to the backup -- you'd need a lot of potential disk space for every file copy, rather than just the occasional exception.

We don't erase the backup first -- we copy over it. (The OS is going to do the erase in its own file copy function, or perhaps just replace the data portion and leave the file in place.) But, the end result is the same.

But, really, this is hardly the main case to worry about. There are lot of similar issues that could occur, including a failure of both the backup media and source drive, and rotating a backup -- or, as I suggest in the guide, three -- is the best way to reduce the probability of this kind of thing hurting you to near -- but not at -- 0.

[cslloyd]

Ok, I understand. But maintaining one or two rotating backups requires a lot of storage capacity too. I guess to rotate a backup file you have another one (or two) copy(ies) of the backup file in a separate partitions(s).

But I am willing to take the chance with one backup copy and the comfort I have with the clarity and simplicity of SuperDuper. I formerly used Retrospect and it has the world's worst interface. I rarely felt comfortable with it so I abaondoned it in favor of SuperDuper.

[dnanian]

Yes, you can do the rotation with physical disks -- which gives you isolation from failures on the backup device -- or with partitions, which does not, but helps to protect from the kind of trouble you mention, or a more common case, where you delete a file you didn't realize you wanted to keep until you'd already overwritten your backup.

These things are rare, and it's basically up to you how much you want to isolate yourself from their potential effects.

But, really, drives are cheap these days, and your data is not, for the most part. $100 or so is a small price to pay for physical rotation, should that be something you want to do.