Solved, by plugging the 3.0 usb flash into the 2.0 usb slot. (there are 2x3.0 slot and 2x2.0 slot at the back, and 2x3.0 slot at the front). So...
I followed the steps to make the usb boot img on SanDisk UltraFit USB 3.0 Flash Drive. https://www.openos.org/threads/dsm-6-1-3-15152-2017715.29/...
用逗号分隔用户名