Drive content disappears after a few hours of usage

I keep my photos on a WD external HD. When I plug it in, the photos are listed and usable, but after a few hours of work, the content disappears and cannot be ejected. it must be force quit. Any ideas?


Please check this link-

For safe ejection follow this link-

Please check this link too-

Thanks for references; I found out that I had to keep unlocking the drive when the content disappears and then it reappears.
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