Will a bad sector cause my backup image to become invalid?

In this excerpt from
Answercast #65, I look at the way backup images are formed and how they can
potentially restore data from bad sectors.

Restoring from an image

Absolutely!

One of the reasons that the term “image backup” is confusing to people is
that there are, in fact, two types of image backups.

Complete sector-by-sector image

One type (and this is not the type that we typically do) actually copies,
sector by sector, every sector on the hard drive – whether it’s used or not.
That creates a complete “image” of the hard drive; of all the data on the hard
drive and whatever was also in all of the free sectors; all of the unused
portions of the hard drive.

That could potentially (when restored) have some difficulty restoring to a
sector that it had been able to read from when making the backup and now
could not write to when restoring the backup.

But, as I said, that’s not the kind of backup we typically make.

Files and folders image

If you’re using Acronis, if you’re using Macrium Reflect, if you’re using
any number of different backup tools, an “image” is actually an image of all of
the files and folders on the disk, regardless of where they’re laid out. In a
way, its kind of like a “super copy” of all of the files and folders.

It’s very close to using something like XCOPY to copy all of the files and
folders. It’s not… and XCOPY is not a suitable replacement for a
backup; I want to be clear about that. It’s not a suitable replacement for an
image copy because there’s more going on.

But, the concept is that what gets stored in the backup image is the files
and folders that were on your machine; not where they were on the disk they
came from, where physically on the disk they came from.

The information about what sectors they were located in isn’t part of
that kind of a backup.

What that means is:

• The backup is a lot smaller because it’s not backing
  up the unused space; it’s only backing up things that actually have
  information.

• When it comes time to restore that image, it can restore to pretty
  much anywhere on the disk it needs to.

That’s one of the reasons that an image backup of this kind is so commonly
used when you’re changing the size of your hard disk; because that way, not
only are the sectors all rearranged a little bit differently (or potentially
rearranged a little bit differently), it’s a completely different sized hard
disk. The image can still be restored to that drive even though it may have
completely different physically characteristics.

So, I don’t think there’s anything to be concerned here.

When sectors go bad

If you do end up with a bad sector, you may end up at some point with a
backup failing. In other words, the creation of a backup potentially
failing because the backup software was unable to read a sector. Some backup
software will fail at that point; some will continue and simply notify you;
but the point is that that’s the point at which you might first detect
something having gone wrong.

But, that should have no impact on your ability to restore a backup image
that was successfully created to another hard drive or even to that same hard
drive if a sector is eventually marked as bad and no longer used.