The first drive you plug in will be mapped to usbshare1, the second usbshare2 and so on.
I've found that after updates the ordering can get confused where usbshare1 will become mapped to usbshare3. This is due to some confusion in the usb mapping file.
In order to rebuild the order you will have to follow these steps:
- Enable ssh or telnet accessibility and remote in
- Move the mapping file
mv /usr/syno/etc/usbno_guid.map /usr/syno/etc/usbno_guid.map.old
- Plug in the drive you want to be mapped usbshare1 first, then follow the order until you hit the desired ordering