The .sav files cannot be used for anything except what Virtualbox wants to use them for under the hood. You can't manually fix this with a .sav file.
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Question, though:
If you were unable to copy the VM to the new disk, how have you been running it? Have you been running it from the old disk? Wouldn't you have had to be able to mount the old disk to run the VM? Seems there's something not coming through here somewhere.
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Curiously, the web-search I did lead me to an article by Microsoft for getting into Safe Mode on Windows 10 which did not mention using F8. Not sure why you'd try it. It did mention F4 & F5, but only as keyboard shortcuts to use after you'd got into Safe Mode. So, not sure why you'd try those either. (F8 used to work on XP, but not on 10.)
Try that web search again. The intent is to get Guest Additions uninstalled to see if they are somehow interfering. I doubt the host OS is involved, especially since another 10 VM is working. You did try making a new 10 VM as well, right?