From the drive upgrade instructions:

Copy the files from the second drive to the first drive:
cp -v /drive1/fids/* /drive0/fids (Enter)


I'd change the second line to

cp -avfx /drive1/fids /drive0 (Enter)


Now, to get a consensus on exactly which flags should be given might be tricky.
Minimalistically, you should in any case where you haven't done weird stuff like mounted some other partition inside the /drive1/fids directory or have a non-empty /drive0/fids, cope with the -rv options.

-avfx would be more robust, managing any weird file types, overwriting preexisting files, excluding any mounts, etc.

I played around a bit with this and can't seem to find a case where it doesn't do what I expect this to do; copy /drive1/fids and everything inside (except stuff on a partition mounted inside) to /drive0/fids, overwriting anything that already exists in the /drive0/fids directory (and below) with whatever has the same name in /drive1/fids.

Any one else?

/Michael