Synchronize problem

Recently got a new mac mini so performed a 'back up' via Library >> Back up Library 

Copied the two files (one folder and one zip file) onto my external drive.

Then copied these files to my new mac's HD, downloaded fresh version of Rekorkbox, gone to restore library and everything at this point seems fine.

Gone to sync with a USB stick and external drive in the usual way and now get the following message:

"This device is synchronized with another computer. Are you sure you want to synchronize this with this computer? If you choose 'Yes', the settings of the synchronisation with be erased." 

When I choose 'yes' the sync starts as usual and transfers everything over...so far so good.

But....after the sync is complete and I go to press the sync option again, I get the same message and after selecting yes it syncs all over again, i.e taking 20 mins.

All I want to do is sync my usb stick and ext hardrive once to get everything on, then as and when I add tracks it should just sync up those additional few tracks as opposed to starting the very long sync everytime after seeing this message.

Can anyone suggest what I can do or what I'm doing wrong?

When the message says 'This device' is it referring to the ext hardrive I'm trying to sync the library with?

Is there a way to un-sync the ext HD with 'the other computer' as the message states?

Many thanks.