SOLVED problem 1! I can now play all tracks with datafile on external HD running on laptop 2. Managed to change user account name in regedit so my 'music file' on C: is the same name on both laptops and all works fine.
Regarding my other questions i would still like to backup this datafile on both laptops in case the external HD goes down. How is this done? I presume whenever i add new tracks to my rekordbox collection the datafile in my external hard drive backs up automatically. I just need to ensure the original tracks i uploaded are on both laptops in same file location for the datafile to read them whenever i plug the external hard drive in. Then obviously make backups of the datafile also, in case of external HD failure.
One other thing after reading pulses comments as below:
pick one database and make that your "primary." It should also be noted that you CAN maintain multiple databases (one internal and any number of external), and you have the ability to switch between them at any time simply by visiting the Advanced Preferences and selecting a different drive from the drop-down (5)
Obviously once the datafile is moved from C: to an external hard drive how can i then use 'primary' or create any number of external (sort of going back to my question regarding backing up datafile).
Hopefully somebody can explain the finer workings
Fryer