Copying large files from attached HDD to NAS FAILS

Hi, new here and to NAS. Just getting mine setup, 8gb. I attached my older WD external HDD via USB to copy it’s contents onto my new NAS.
It will copy for awhile and then fail with error code -50.
My computer system is a mac and so that is how things are formatted I guess.
I’m copying my dragging from the HDD folder to the new NAS folder.
Is there a better way?
file sizes are a few hundred mb.

Is there a size limit? Was hoping I could drag it over and even if it says it will take 2 days, it will do it. I’m not reading or writing to the disc otherwise.