Generally when copying a large amount of files/data from a local network computer to a local network My Cloud its best to have that computer connected via Ethernet wire to the same Gigabit capable router/gateway/switch as the My Cloud. One can connect the USB flash drive to their computer and use Windows File Explorer or Mac Finder to copy the files from that flash drive to the My Cloud. If one connects the USB flash drive to the My Cloud then one may experience a slower transfer speed when using Windows File Explorer or Mac Finder as the copied data flows from the flash drive out of the My Cloud to the computer doing the copying and back to the My Cloud.
Another option is to connect the USB flash drive to the My Cloud’s USB port then use SSH to trigger the copying of files from the USB flash drive to the My Cloud internal hard drive. One can use either a command line SSH program like Putty or a SSH GUI program like WinSCP to copy the data from one location to another.
My Cloud: Access Using SSH (Secure Shell)