So, in such a case would
we then have an opportunity to make a 2nd transfer request asking that the things
which had been 'left behnd' could be moved?
Or, is a 2nd attempt to
transfer any 'left behind' things not something that Groups.io is able to do?
Yes, you can.
If you go to the "In Process" page for the completed transfer there's a red button at the bottom that will allow you to delete the old transfer status. Then you initiate a new transfer from the same Y!Group to the Groups.io group, selecting only the sections that were missing.
If a section was only partially copied (e.g. some files copied, some not) I believe the automation will automatically skip any content that is already present in the Groups.io group, but I haven't tested that (and haven't had to, knock-on-wood).
The idea here is that some of Yahoo Groups' failures have been sporadic, so it is possible that a second attempt may be able to copy some items that hadn't been accessible the first time. If you can now access (using the normal Yahoo Group web page) any of the items
listed
as not transferred, then there's a good chance that the transfer agent will be able to now also. If you can't, maybe wait in hope that you can later.
Also, if you can now it would likely be prudent to go ahead and download them manually (if there aren't so many as to make that impractical), just in case.