When using ddrescue to scan an HDD and it displays the number of errors, how can you know if any given error count is acceptable, and the drive still usable?

Is the drive considered unusable if ddrescue is unable to read a certain number of sectors ?

If so what is that number ? Does it depend on the capacity of the drive ?

If you are using ddrescue on a drive then the reason you're probably using it is because the drive already has errors and you are having trouble accessing it. The fact that it gives errors means that there's an issue (small or large) with the drive. I would never trust my data to a drive that has issues. With drives being so cheap these days just get yourself a new one and make sure you have backups backups backups.

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Death is nothing, but to live defeated and inglorious is to die daily.
Data Recovery Cape Town

I agree completely

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You don't have to backup all of your files, just the ones you want to keep.

OK I see.

One more question.

Is the error count in ddrescue a count of INDIVIDUAL unreadable sectors, or "blocks" of unreadable sectors.

"Press F4 again to start the scan. MHDD scans drives by blocks. For IDE/SATA drives one block is 255 sectors (130560 bytes). "

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Death is nothing, but to live defeated and inglorious is to die daily.
Data Recovery Cape Town

Yes or even a combination of the two. The answer depends on how you are using ddrescue and when in the process you look at that count.

Think about the default algorithm used by ddrescue (which you can influence with command line options, and often need to do so for best results). You'll see that during that process, ddrescue has different information available to it, at different times.