Hi -X- and thanks for the suggestion. While I tried your script and it did work, I'm left with even more questions now. Previously, I had prefixed each update file with a number representing the order in which it was (automatically and successfully) installed by Windows Update. This was to avoid issues with the several updates having invalid or missing "Build Date" meta-info - which I was surprised to see UDC does not appear to address. When running your script, I simply stripped these numbers to restore the original names, then dumped the files into the UDC folder.
None of the files which caused errors was altered, replaced, or removed by UDC, so the errors must have been caused either by the order I was adding them in, or by one of the dozen or so updates UDC moved to its "Obsolete" folder. Neither of these explanations holds up very well, because of the renaming convention mentioned above, and also because all of these "obsolete" updates are still being downloaded by Windows Update on a fresh non-slipstreamed install.
Edited by mclone, 02 September 2012 - 04:25 PM.