Yesterday when I attempted to install the AutoCAD 2011 service pack (er, I mean “update”), I encountered the classic UI paradox where you’re forced to choose from a set of equally undesirable choices, this time with a bonus.
I received error 1321. My choices were to retry or cancel. If I chose to retry, I got the same error again. If I chose to cancel, I got a bonus question: it asked whether I really wanted to cancel. I had to lie and choose yes, otherwise I would be trapped in an infinite loop.
The error message indicated that the cause was the installer’s inability to write to the original installation media. I’m not sure why it wanted to write to the original media. The only way to apply the service pack was to uninstall AutoCAD, create a temporary copy of the original media with write permission enabled for the copy, then reinstall AutoCAD from the copy and apply the service pack afterward.