[gd123]

As I understand it, you had 2 External HDs with KMAs and one was designated as a BACKUP Drive.

One Ext HD failed, so you had to replace it and COPY the files from the ONLY GOOD External HD to the NEW External HD.

Now, it is possible that, upon connecting the NEW Drive to your computer for the first time, your computer assigned drive letter "X."

Then, possibly, you changed the DRIVE LETTER to "F," in keeping with the way your Drives were assigned prior to the one Failure.

Drive letter of "F" assigned to each individual drive works fine only when ONE drive is connected to your computer at any given time.

Now, when you talk about copying files from ONE Ext HD to the Other, you would have had the TWO DRIVES connected to your computer at the SAME TIME.
It is not possible to have the SAME Drive letter assigned as "F" at this point.

So, I suspect that, when both drives were connected at the same time, YOUR OS assigned a Different DRIVE LETTER to the drive that you thought was "F."

This is the “ONLY” reason way HOSTER would not be finding the KMAs on that DRIVE.