The system gets its fonts and desk accessories from the default drive. The default drive is (a) the drive you booted from, or (b) the last drive you ran an applicaiton from.
To transfer a picture from one Scrapbook to another, then, simply do the following:
1. Boot up with the disk you want to get the picture from.
2. Open the Scrapbook, copy the picture, and close the Scrapbook.
3. Eject the disk and insert the disk you want to copy to.
4. Run an application on the newly inserted disk.
5. Open the Scrapbook and paste in the new picture.
Step 4 is the one many forget to perform. Even though the Finder is sometimes a little clumsy--asking for the old disk back at surprising times--it's not so bad once you understand that you must run an application to make the system "switch horses."