Is it possible to copy files from a USB drive to the My Cloud device via the USB port? Or is the USB port used just to expand the capacity of the My Cloud?
It is possible but not recommended; the User Manual recommends connected the USB drive to a PC, and copying files to the MC via the PC.
Yes you can copy files to and from a USB hard drive or USB flash drive attached to the USB port on the back of the My Cloud. Note however that if copying a large amount of files to the My Cloud, that it is generally recommended NOT to use USB port on the My Cloud, rather to attached the USB device to a computer, connect that computer to the same router/gateway/switch/hub via Ethernet cable as the My Cloud, then use either Windows File Explorer or Mac Finder to copy the files from the USB device to the My Cloud.
It should be also noted that users have reported slow copying speeds at times with certain USB 3.0 USB devices connected to the My Cloud USB port.