Disk Image Problem?

As for backup, I have a Time Capsule, I use SuperDuper to clone my HD every other week, and weekly I separately back up the image to a thumb drive. Additionally, periodically I copy the opened image contents to a thumb drive (unencrypted, so stored in a safe, along with the clone).

Considering all that you have no reason to be concerned. There is no reason to be concerned unless you had one and only one backup.

Don't use clever non-Apple disk "doctors" or "cleaners". There is a high correlation with the use of such things and premature device failure. Don't rely upon any product or utility to predict the onset of disk failure either. They don't work and are proven to be unreliable.

When a disk drive fails, throw it out. Purchase and install a replacement and restore its contents. If that prospect causes you the slightest concern, then reevaluate your backup strategy.

I use an encrypted disk image to store sensitive data.

It's not clear to me where that disk image resides. Obviously it's not a good idea to store it on the same device in which the sensitive data reside.

John, the image is an encrypted sparse bundle that lives on my Mac hard drive. I did this long ago, so that in the event my Mac was stolen the evildoer most likely couldn't get into my sensitive data. Since it is encrypted, I think it is fine to place it on the drive. Or am I missing something?

You're better off just using FileVault. That's what it's designed to do. Without its password a FV encrypted startup disk is utterly useless to anyone.

BTW, are you suggesting that the SMART parameters reported by the Apple techs are unreliable?

Yes they are. All I have to go on is my own personal observations and I have never attempted to perform anything resembling an extended long term analysis with the scientific method though.

Fortunately others have. Quoting from Failure Trends in a Large Disk Drive Population:

".... we find that failure prediction models based on SMART parameters alone are likely to be severely limited in their prediction accuracy, given that a large fraction of our failed drives have shown no SMART error signals whatsoever."

Besides, Apple has little or no interest in hard disk drives any more. As far as they're concerned they might as well already be obsolete.

And, yes, I generally avoid the various third party apps hyped by the IT media. I guess everyone needs to make a living.

Not at your expense.

It's been my experience that all recently manufactured spinning hard disk drives are less reliable than in the past. One day Western Digital may get a bad reputation due to premature failures. The next day it's Seagate, or Toshiba. With the possible exception of Hitachi (HGST) they're all the same disposable junk... and HGST has since been sold to Western Digital.

It has been posited that the 2006 era's lower density hard disk drives were more reliable than the multi-TB models more commonly installed today, but there is no way to prove that. I attribute it to the general race to the bottom of all electronic manufacturers in their zeal to lower costs at the expense of everything else. HGST was always more expensive than the competition, but all that accomplished was lower sales volume.

Apple no longer wants to play that game. Solid state (flash) memory is all they're interested in. Of course it fails too, but for different reasons.