When using Windows auto-update, it seems to save the install files on my large external hard-drive, rather than my Windows c: drive. I'd rather not have all that stuff on my external drive - is there any way to force it to only use c: for windows update files?
The windows update files are downloaded to your c drive, i.e. the same drive windows is installed. They should be in %systemdrive%\windows\SoftwareDistribution
What i think you mean is that you end up with folders like: X:\ee9bb670ca8d1b9684e57427c9bd30 on your external drive?
From what i understand, the behavior of most windows security updates is to extract themselves into a randomly generated folder, on a local disk drive with the most free space. Not sure if there's an easy way to fix this other then having more free space on your c drive then your external HDD.
Just a guess here, but if you can flag the drive as removable, maybe windows wont try to extract the files there?