For anyone using xpenoboot on a USB stick, you may be aware that this boot-loader USB stick is mounted and visible within DSM.
On Xpenoboot, it is by default enumerated as /dev/sdu. The partition /dev/sdu1 is mount on /volumeUSB1/usbshare.
On a real Synology system, there is also a USB bootdrive embedded inside the chassis. However, I believe there is some check for specific VID/PID during the boot process that prevents a specific Synology USB bootdrive device from being enumerated.