Getting error code -8058 when I try to copy files from iCloud drive to flash drive

Hi, I have a lot of files in iCloud drive, over 2Gb, all neatly sorted into folders. Although I back up my Mac using Time Machine, I also like to keep a separate copy of the folders/files on a USB drive, just for belt-and-braces assurance. I have regularly done this but last successful back up was 6 months ago. Since then, when I try to copy/paste I get this message:

"The operation can’t be completed because an unexpected error occurred (error code -8058)."

Why is this? I've searched Help and other previous questions but haven't seen an explanation. I have reformatted my USB etc.

Can anyone help?

Thanks