The answer was that the .git folders were missing. The issue had nothing to do with when the folder was copied, but how.
The various folders in which git worked fine were pushed from the old drive to the new via Windows Explorer; the .git files got pushed with them.
The offending folders, however, were pulled off the old drive onto the new, via DOS Xcopy, I think, which skipped the .git folders.
Thanks.
solved Git repos on new hard drive [closed]