When you connect a USB device to your Synology diskstation a share is created with a sequence number. The last number assigned (and the device id it is assigned to) is stored in the file /usr/syno/etc/usbno_guid.map.

You can delete devices you no longer use and assign your own numbers here to the USB shares. This is very usefull when you have backup tasks that backup to a specific USB share (like I have).