[Bug-ddrescue] When will another pass help?

[Colin Berry]

Like most people I am turning to ddrescue after a hard disk failure. (Not the
best time to learn something new.) The good news is that it was my kid's
hard disk, so mostly a teaching moment for me. Already been through the
"importance of backups" speech. (And the do as I say not as I do talk that
goes with it.) Now for trying to fix it. 

We got ddrescue and read all we can and have a couple of questions.  If we
tell ddrescue to abandon rather than fix a bad area with -n, why does it
take so long? Is this the same length of time that a regular dd would take
were the disk healthy?

Where does the advantage of a second pass come from? Does it presume that
the problem is elecrical or hardware related and thus not static over time?
Or would there be an advantage if the problem were purely media related
(jelly stain on a cdrom) and therefore a stable reproducible error?

Can we tell by looking at the logfile of the first pass if another will
help, or is it better to run multiple passes and evaluate for diminishing
returns?

btw Since this is largely and educational thing, links with the answer are
just as good (maybe better) than the answer.
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