I think you don’t need to “attach” a USB device to sys-usb! It is available directly. Try running lsusb in sys-usb after inserting the USB device. And sudo dmesg will show the details, like the device name (ex. sdb) and partitions.
That’s right, the error message is complaining about the attempt to attach a device from sys-usb to sys-usb (which would be a connection that “loops back” to where it started).