After a week of using "cp" on the command line, I'm confident in saying this is a viable work-around. No issues whatsoever using "cp" with single or multiple files.
This is obviously a Finder issue and has nothing to do with the ability to actually copy files.
It could be related to the hidden files that AFS is dead set on putting everywhere it can. That might explain why you can copy using Finder sometimes and you can't other times (timing of the reading/writing of those hidden files???) - but I can't be sure about that.
I disabled creating the .DS_Store files (using "
defaults write com.apple.desktopservices DSDontWriteNetworkStores true"
) but that did not prevent the issue. I also turned on Blue Parrot (which tries to do the same thing) but that made no difference either (nor did killing that process).
For me, the issue only occurs when using Finder to copy large files from my local hard drive to a network drive - those that take long enough to copy that the progress bar appears. I never get the error when copying small files or even large groups of small files. I don't get the error when copying large files locally. I don't get the error when copying large files from a network drive to my local hard drive.
For now - I'll use "cp" to copy large files to a network drive.
C'mon Apple - fix this! It's been around for a couple major OS upgrades already!