my two cents. (first, I never used the file manager on Asustor nor on QNAP ... I have the two brand ... and work for developing package (n the past QPKG for QNAP and now APKG for Asustor) but I have always some NAS from this two brand (and some others
) .If you have a minimum skill in command line tools ... better to move internally is to use command tools in a ssh terminal session.
cp, tar, etc.
If it's not enough, you can try to install Entware APKG adding more than 2000+ packages to your NAS, including full GNU version of tools (Asustor as QNAP used busybox a reduced version of command.
Philippe.
If you prefer file explorer, as Orion write ... report to support.
to ADM 5.0 
