Can't open backup of expandable master document

[Unknown]

I've backed up a master document and its subdocument files to another
drive - eg usb memory stick.

When I try to open the master, WP hangs.

I guessed it was looking to see if the sub doc files were where the
master said they were, and couldn't find them.

So I stripped out the codes at the end of the master doc but the
problem persists.

How can I back up my master doc so that I can open it?

Suggestions gratefully received.