OS X: "The folder “Backup” can’t be opened because you don’t have permission to see its contents."

This was a little hard to find the answer to. I was pretty sure that it was a permissions problem, but it wasn't clear exactly what the permission was -- I was looking at ACLs, and UNIX permissions, but it didn't seem to be working.

I put in the error message into Google, and got a fairly useful MacRumors forum. What finally did the trick was a "chflags" command in the terminal:

cd /Volumes
chflags nouchg Backup

Where the disk / mountpoint that I was having issues with was called "Backup". Macrumors suggested adding a -R(ecursive) flag to the chflags command, but I didn't do that. Not sure if it's going to cause problems down the line or not. But for now, it's working.