You're talking in general terms, right? Well, first comes Scandisk. See what
it says. Simple things, like wrong dates or mis-reported freespace, go ahead
and fix. Anything more serious, ask an expert. Based upon the finding of
Scandisk, the expert will suggest other tests to see what the real problem
may be. There are other tools that are in the same category as Scandisk
(Norton Disk Doctor, for example) that may or may not be "better", but
AFAIC, they should be dealt with the same way.

When it comes to testing the physical drive, including the one known as
Scandisk "thorough", which tests physical clusters, that's a job for the
tools provided or suggested by the drive's manufacturer. If there is any
hint that clusters are starting to go bad, you want to immediately cease use
of the drive until you can retrieve as much of your data as possible.

The danger with all drive tests is that the test and/or the fix will itself
do more harm. File system repairs, for instance, may cascade, with one
"repair" invalidating another FS entry, which when "repaired" invalidates
another entry, etc., etc. With physical problems, the primary danger is that
a cluster that is only marginal may get marked bad and thus unreadable.

In short, other than the simple, common errors like free-space, invalid
names/dates, any error is almost certain to be a serious one, and you should
*FIRST* make sure you've recovered all the data you can off of the drive
before proceeding with attempts to repair. And, when it comes to file system
errors, there are often things an expert can do, manually, that do a far
better and safer job of it than any automated app. Of course, if you've
retrieved your critical data and "don't have anything to lose" by trying an
automated fix, go right ahead. Other than Scandisk, the only other one I've
used regularly is Disk Doctor (2000 era), which I consider better than
Scandisk. Don't know enough about any others to comment.

Yeah, that'll cream a backup/image procedure every time, <s>.