How to fix iTunes downloading IOS update but not installing?
I do not know whether this is general or something unique to me, but here's what seems to be happening here:


When iTunes is asked to update the iPad or iPhone, it downloads the update but doesn't install it.  Usually there are no diagnostics, but on one occasion there was an "error 4000" when the download completed.
After the download, putting the device in DFU mode and launching iTunes causes iTunes to install the downloaded update.
Trying to do DFU mode without doing the download first fails.


None of the "normal" update methods did anything, though they have worked on these devices on previous updates.