The problem should be a macOS memorize issue. Once you install USB2 device to USB3 port, this port will support USB2 only before next start up.
Areca had reproduced this behavior in our lab with Mac Pro 2013.
Please follow these procedures below to be able to link with USB3 mode
1. un-mount the raid box
2. un-plug raid box
3. shutdown Macpro (not reboot)
4. power on Mac
5. connect raid box after OS ready
6. check the USB device list to see if raid box been linked with correct speed.
To avoid this situation, do not connect raid box to USB3 port which had ever connected USB2 devices before.
Tags: ARC-8050T3U-8, Mac Pro, macOS, USB 2.0, USB 3.0