Error stopped the image file transfer

Hi all! I was manually copy-pasting a Photos library from one MacBook (10.14.6) to flash drive to free up the space. However, I encountered an error that stopped the transfer. Moreover, since these files were on desktop, I tried to move them back into the original folder and that failed as well. The size is 15 GB and I do not really see why would these failures occur at all in the first place. Did anyone experience a similar problem?

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