It is one of the options on the unRAID boot menu.  If you have a monitor+keyboard on your unRAID system then you can select it when booting the system.   If you run your unRAID system headless, then you can edit the file syslinux/syslinux.cfg that controls the boot menu on the USB drive and move the 'default' entry to the section for Safe boot.  If you then reboot the server you will be in Safe mode.