Since you've already tried reinstalling apps and rebooting, it might be a deeper issue. Water damage could definitely be a factor, especially if the internal components responsible for video playback (like storage or processing chips) were affected. You might also want to check if the issue happens with all videos or just specific ones—if only some are affected, they might be corrupted. In that case, you can try repairing them using the methods outlined here: https://news.macgasm.net/tips/fix-corrupted-video-iphone/. If none of these solutions work, it may be worth running an Apple diagnostic or getting your device checked by a professional.

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If your "My Book" has an external power supply, ensure that the power adapter is working properly. You can try plugging it into a different power outlet or using a different power adapter with the same voltage and amperage. Another options and solution you can find here https://www.handyrecovery.com/recover-data-from-western-digital-hard-drive/ . This blog i've been following for some time now and found tons of great tips there.