Greetings!
Well, it depends on if you are removing the master (drive0) or the slave (drive1). Your original 10 was built using the drive builder, and has the boot software on it from the emplode load. The 20 you added was probably also built with drive builder, but may (or may not) have the software loaded. If you replace the boot software on the 10, you need to make certain that at least one of the drives has the player / boot software on it.
Apart from that, it is very easy. All of the music and data files associated are stored in the /driveX/fids directory. The /drive0/var directory holds the database files (database, playlists, tags). These files are automatically rebuilt on every sync, so they can be destroyed without too much concern.
If the drive you are swapping has an empty fids directory, you can swap the drive without any issue. If any music or data was loaded on it, you just need to copy the hex named files from the fids directory elsewhere (the other drive's fids directory) to keep your music in tact. In this case, delete the database files, reboot (it will be slower as it tries to regenerate the files on the fly) and resync.
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Paul Grzelak
200GB with 48MB RAM, Illuminated Buttons and Digital Outputs