Comment 2 for bug 2039786

The issue is with the onboard_usb_hub module.

During boot, when the onboard_usb_hub it causes all USB devices to restart, including the disk Ubuntu is booting from.

rpi kernel: usbcore: registered new device driver onboard-usb-hub
rpi kernel: usb 1-1.1: USB disconnect, device number 3
rpi kernel: lan78xx 1-1.1.1:1.0 (unnamed net_device) (uninitialized): Failed to write register index 0x00000010. ret = -19
rpi kernel: lan78xx 1-1.1.1:1.0 (unnamed net_device) (uninitialized): Registers INIT FAILED....
rpi kernel: lan78xx 1-1.1.1:1.0 (unnamed net_device) (uninitialized): Bind routine FAILED
rpi kernel: usb 1-1.1.1: USB disconnect, device number 5
rpi kernel: usbcore: registered new interface driver lan78xx
rpi kernel: usb 1-1.1.3: USB disconnect, device number 6
rpi kernel: sd 0:0:0:0: [sda] Synchronizing SCSI cache
rpi kernel: sd 0:0:0:0: [sda] Synchronize Cache(10) failed: Result: hostbyte=0x01 driverbyte=DRIVER_OK
rpi kernel: sd 0:0:0:0: [sda] Stopping disk
rpi kernel: sd 0:0:0:0: [sda] Start/Stop Unit failed: Result: hostbyte=0x01 driverbyte=DRIVER_OK

I blacklisted 'onboard_usb_hub' to workaround the issue.
echo "blacklist onboard_usb_hub" > /etc/modprobe.d/blacklist-usb_hub.conf