Install Midnight Commander on OMV
apt-get install mc
in Webgui just mount the drives
over ssh or console; start mc navigate to each /media/uuid/ on right side and on left side > source and dest. then F6 to move.
or use mv on console
or whith Plugin: mount 1 empty ext4 drive make a shared folder then install USB-Backup Plugin and configure it to synchronise from ntfs drive to shared folder then Start Job
yes it can copy not only from USB also from internal mounted drives
Copy most data to largest drive under Windows,
you can also copy over SMB or FTP to your OMV.