When you connect a USB disk to a Synology NAS, the NAS assigns it a disk number, which forms part of the name of the mount point for the disk.
The first disk you plug in will be usbshare1, and then if you have this disk plugged in and plug in another one, it will be usbshare2 and so on.
DSM keeps a file with serial numbers for each drive and what USB disk number that disk was.
This file is /usr/syno/etc/usbno_guid.map
This is a simple text file, and in it is a mapping from disk number to serial number.
I needed to change the number of a disk, so that each of the two external backup disks would mount at the same path. Things got mixed up however, and one disk mounted at usbshare2 and the second one mounted at usbshare3. I needed them both to be on usbshare2.
I was able to ssh into the NAS, unplug the disk that was mounting as disk 3 and edit the file, changing the number at the start of the line from 3 to 2. Then, when I plugged the drive back in again, it mounted at the correct path.
the file was originally like this:
I simply changed the line 3=”… to 2=”… and that changed the mount point for the disk when I plugged it back in again.