Which programs have you used to try to open them? sometimes a different program will have no probs.
Do the Thumbnails view show ok? Or it may even be an 'ownership' problem and you have to take ownership of the files on the ext HDD
Huawei P10 review
1995-2015: How technology has changed the world in 20 years
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New iPad, iPhone SE & Red iPhone 7 on sale now
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.