File Destruction

[DBF]

I have a Problem, when I want to see my Pictures in "big Size"
The Setup is: Mac Mini 3GHz / I7 / 8GB / 1TB and a Sony VW520ES - 4k - Beamer - Phocus latest Version.
The fff-Files are on a external Hard Disk - non SSD with USB3.
After about 10 Pictures the File is not opened - only "Noise" like an old TV-Set without Antenna-Signal had is visible.
I can go to the another Picture - ok.
But: The last File is "destroyed" - the "Noise" is saved.
How can this happen?
How can I avoid it?

[NickT]

Are you looking at the images at 100%?

[DBF]

Yes - I use "Shift + æ + F"