Well, so far so ... I'm not sure.
fix-disk, run with the new options, was left running for ages; after which is has reduced the current_pending_sector count from 152 to 24. But in doing that it found (and says it successfully re-wrote) 9,411 bad sectors. Yup, that many. In the end I stopped it, so I could see if the box was still working (and use it) - seems it is. So there are presumably more to find if I run it again.
Now, I know the raw SMART data are manufacturer's secrets, and they aren't letting on what they really mean (not Seagate, at least). And I can't find any straightforward description of how these modern disks work and are tested. But as far as I can make out:
This long disk test, in its scan, reads every sector in LBA order. So the data it reads is what the box wrote in a file, or - for sectors it's never used - it's data written during formatting. What kind of formatting did that (i.e. write every sector) I don't know.
The Reported_Uncorrect count hasn't budged, from which I conclude that this only shows in-service read errors, not ones during this self-test. Raw_Read_Error_Rate and Hardware_ECC_Recovered are equal; allegedly that means no unrecoverable errors - I don't understand that.
Reallocated_Sector_Ct is till zero, confirming that all these sectors that read as "bad" on test then read OK after being written. From earlier tests it does look as if the errors are in unused space, with files just recently reaching the lowest LBA. No rewritten bad sector has been found as bad a second time.
So something has made a lot of the upper (high LBA, or inner?) part of the disk report errors, in areas never written to since the disk was new. But there are also errors in files written within the last two weeks. That's a puzzle, if the problem is in the write process.
Back in post #1, I said "from ca. 3 months ago, radio screen display behind the screensaver "wandering clock" flashes randomly when it should be blanked." I wonder if that might be related after all. For example, if a power supply voltage was out of spec., or not regulated, what might that do to the disk? Are they easy to find and look at inside?
With over 10,000 LBAs for bad sectors, there is scope for a lot of analysis. But only in LBA space - is there any way of relating that to physical position on the disks? I know the head/track/sector numbers are pure fiction, and the manual doesn't offer anything more real. There are other parameters there that allow estimates of (for example) sectors per turn, but the answer does not make any sense of the pattern of errors.