OK, but that's what's happening.
Maybe SuperDuper is passing a bad argument or something? But FWIW, SuperDuper has copied lots of other items containing quotes with no problems. (Maybe there's a difference between filenames and foldernames with quotes?)
Here's the copy-and-pasted terminal command that worked:
ditto -rsrcFork /Volumes/Macintosh\ Data/60\ GB\ -\ \"OK\"/Developer/Documentation/QuickTime /Volumes/Hard\ Drive/test
In other words:
ditto copy
problem folder =>
test folder on Hard Drive
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