Folders become empty when copying

I've noticed a few empty folders, or entire sub-folders missing, when transferring my game's project folder to an external drive (in my case, either a SanDisk USB flash drive or external hard drive, each with dual USB A/C capability; models SDDDC2-128G-G46 and SDSSDE61-1T00-G25), which I do for backup and/or to test/develop on another laptop (e.g. Windows to Mac or vv).

The disappearance apparently happens during the transfer to the drive - as in, when I noticed files missing after...

Folders become empty when copying