Windows 10: Anniversary update fails, leaving system unbootable

Perhaps someone can tell me what went wrong here.

I have a bottom-rung Dell Inspiron 330s that I inherited from a family member. It shipped with Win7 Home, and while it was eligible I had updated it to Win10 1511. Very little if anything on it ... pretty much a vanilla system. Not even third-party drivers. Every few months I'd turn it on to get updates. MBR hard disk, no UEFI.

Since it's unused at present I decided to try the Anniversary Update as a practice run. Booted the machine to 1511 as usual, everything looked fine. I did the update the same way I had done 1511 on this and perhaps a dozen other machines - copied the ISO to a temp directory on the C: drive, mounted it, and run setup.exe

The first phase of the setup ran to perhaps ~55% (I had looked away for a few minutes). When I looked again, the 1511 desktop was back with a little white alert box saying that the update had failed. No error message or reason.

I checked the update history and there was no error related to this attempt. Ran the Windows Update troubleshooter, which said it had found and repaired a problem with the Windows Update service. Then a message came on the screen telling me that the drive was damaged, click here to reboot and repair.

Despite the chorus of characters dancing in front of me and yelling "***DON'T DO IT!!!***", I "clicked here" and the machine tried to reboot and failed with a message that there is no bootable OS. Tried booting a real 1607 install DVD and it just hung at the blue Windows logo. So I booted a WinPE disk and had a look at the drive with AOMEI.

There were three partitions:
39.19 MB FAT-16 (Dell diagnostic)
21.67 GB NTFS (Recovery, boot)
444.06 GB NFTS (OS, boot)

The recovery partition was unreadable. Ran chkdsk which found errors in the index file; after repair I could get a directory listing. Kept running chkdsk to be sure it found no errors ... looked good. Deleted the Dell diagnostic partition just in case, though I had never read any reports that this caused issues on OS update (it certainly hadn't when going from Win7 to Win10 1511).

Now I booted the 1607 install DVD successfully, and had it repair the system. On reboot I was back to the 1511 desktop as before. Going for broke, I tried an upgrade install using an EXTERNAL drive as the source (my Zalman enclosure with virtual optical drive function). This time the install went smoothly without errors, and the system is now running 1607.

What happened?
Dell partition confusing things?
Issues with mounting the install ISO on a running OS?
Already-corrupted recovery partition which just happened to work first time, every time, until today?

Think I'll wait before attempting another machine...

:)

 

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Your solution will probably be near the end of this thread - but read the whole thing.

How to determine what caused Anniversary Update to fail - Windows 10 Forums

 

Thanks for the pointer.

I take it, then, that the original 444.06 GB reserved partition was too small for Windows 10 and things might have gone better if I had created a 500 GB partition up front.

But on my second try, after simply reparing the file system on that 444.06 GB partition, the install was successful ... why?

(Thinking out loud: maybe when I allowed the 1607 boot disk to "Repair" the 1511 installation, it created a larger recovery partition? Must admit that I was so happy when the install completed successfully that I failed to go back and see what the final disk structure looked like. Will do that tonight)