Not unusual, my drives do the same from time to time. Easiest solution is to close the window and re-open it when you have inserted the 'new' disc.
I think the problem results from RAM not clearing for some reason.
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How to get WWDC 2017 tickets
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.