There are a number of ways to do this.
Create a copy of the document you want to copy the Master page from. Delete all pages except for the one you want (but you have to leave Right and Left alone as FM must always have these two pages available).
Open the new document and the one that you want to import the master pages from.
In the new document:
Import from Document: (select the document with the Master Page that you want)
File > Import > Formats
and select Page Layouts only.
Note that Right/Left always come along for the ride.
Alternative and probably best way to get a single Master Page:
In new document, add a new Master Page and select "Empty".
In the old document, on the desired Master Page, Select All on page (ctrl+A) and then copy (ctrl+v).
In the new document on the new master page, paste (ctrl+v) to add the layout. Note: you may need to rotate the page first if it isn't in the same orientation as in the old document.
The old document(s) (book) has portrait and landscape pages with many background frames.
Ctrl-A only copies what's in one text frame in the old document.
Ctrl-click collects several text frames but Ctrl-V pastes everything into the new page in the new document in ONE text frame.
The old document has many carefully aligned background text frames which I would like to preserve. How can I copy all of one master page in the old document into a new master page in the new document?
To copy everything on the master page, be sure to click somewhere on the page outside any text frame before pressing Ctrl+A and copying.
When you go to paste the copied master page contents into the new master page, the page should be completely blank--if it isn't, be sure to click outside any text frame before pasting.
That should get everything from the old master page to the new one.
Hmm, I don't know why Ctrl+A would not in fact select all items, or what would cause the items that are selected to be either merged or shrunken when pasted in.
You are copying from and pasting to the master pages themselves (View > Master Pages), right? If you were copying from body pages, then the background frames would of course not be selected by Ctrl+A. That still doesn't explain the shrunken & merged behavior you are seeing though.
I suppose this could be a Frame 8 thing--I am still on Frame 7.2, but I would not think this sort of thing would have changed from version to version.