I periodically backup my Core after downloading or ripping new music. I do two backups; one to a Synology NAS and one to a removable USB hard drive.
I am not having any problems with the USB hard drive and had not been having any with the NAS. However the Core now wants to backup some 140+ files every time even though it is completely backed up already. I have checked that the files are not being duplicated in the NAS but it spending an awful lot of time doing whatever it is doing.
Assuming you have tried a full power off restart of your Core and it still exhibits this behaviour then it would be worth reporting this to Naim support as I know the developers had been looking at a similar report to this during beta testing (but I don’t know where they got to in fixing it).
I think I agree. I don’t think it is backing them up again but it takes an awful lot of time not doing it! Because I have to change settings on the NAS before and after the backup, it is a bit of a bore.