You can't export covers through Excel.
You can assign the former letter for the external drive, in Windows it's possible. It is also a good idea to use relative file names in All My Books, you need to check "Use relative path for ebooks and audiobooks", in "Expert options" tab in Preferences. Here you can specify one common folder for them, say, E:\ebooks. When the drive changes the letter, for example, to F: then in AMB settings you need to change this common folder to F:\ebooks and the program will see the files again.
Bolide Software Support Team