I have made some progress in recovering the AF117a coverdisk.
(See this post
It turns out that the disk is not completely overwritten, as I had feared, but the volume name and creation date has been changed and an attempt has been made to copy a file onto the remaining free space. Some of the files on the disk have been damaged, possibly by removing it from the disk drive while it was being written to; hence the checksum errors.
Inspection of the files salvaged from the disk reveals that more than one file is damaged, but the largest of them is not nearly so badly damaged as it might have been.
Recovering the coverdisk contents may still be possible thanks to the inclusion of the uncompressed
disk files on AFCD 33, the cover-mounted disc available with the CD version of the magazine.
I will post further updates on my progress in this thread.