Dear Sony, "Something went wrong" is not useful error information.

TL;DR - if you get the wonderfully helpful can't install (CE-10002-3) because something went wrong for a PS4 game installed in the PS5's internal storage you could try to temporarily remove your external storage device and try again.  Perhaps cancelling the update and trying again after turning the always install PS4 games on external storage option off might work too, especially if an update looks like it's the size of the entire game.

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Today my PS5 decided it wanted to install the 2.06 update for Ghost of Tsushima (PS4 version).  Jolly good, yes.  Unfortunately, after downloading well over forty gigabytes of 'update', the wonderfully helpful system software decided that it can't install the update because, brace yourself, something went wrong. The error code CE-100002-3 is similarly useless.

No amount of re-trying, restarting the console, restoring licenses, freeing up space, etc. would resolve this issue...  just a brick wall of can't install because something went wrong.

Anyway, there's an important clue:  the update should be about a hundred-ish megabytes, not forty-plus gigabytes - that's the size to download the whole game, not just the update.

It rather looks like the stupid system software was trying to install the whole game, and onto an external USB drive.  That sort of makes sense since the option to always install PS4 games onto external storage was turned on.  

The only problem is that I have GoT installed in the console storage.  So why the hell is the thing trying to install a whole new copy of the game on the external drive, then throwing its hands up in despair because something went wrong, presumably that installing different versions of the same game in two different places would be messed up.

The final clue that that may well be what's happening is that when I told the PS5 to safely remove the external storage, it sulked because for some reason the GoT 'update' was being installed on it.  Only it wasn't, of course, since it can't install because something went wrong.  Setting the always install PS4 games on external storage option to off at this point didn't help.  Maybe because the console was hell bent on installing the 'update' (which is the whole game, remember) on there by then.

Safely removing the external storage and then getting the console to check for an update for the game finally worked just fine.

I feel like this is an issue where an actual frickin' error message would have been helpful.  Of course, the system software not being brain-dead when it comes to updating a PS4 game that's been moved to the console storage would be nice... but hey, if we're going to dream that big, I'll have a PS5 remake of Bloodborne a la Demon's Souls, tyvm.