This article explains the reason for the gbtboot messages reported upon bootup.

When Junos OS with upgraded FreeBSD is booting up and receives interrupts from the console during the boot-up process, the boot process may get stuck at the gptboot prompt. The gptboot prompt is reported upon every key entry/interrupt from the console:

gptboot: No ÿÿÿ on 0:ad(0p2)
gptboot: No ÿÿÿ on 0:ad(0p2)
gptboot: No ÿÿÿ on 0:ad(0p2)

This is because the -n parameter is not set in the boot.config in certain releases.

/boot.config
-n       ignore key press to interrupt boot before loader(8)
         is invoked.

The fix is available from FreeBSD version 20180708.145119_fbsd-builder_stable_10.146b4b3 (via PR1371344 ).

The following kernel version doesn’t have the fix because the date is prior to 20180708:

> show version detail | match kernel   
JUNOS OS Kernel 32-bit  [20180310.ba55661_builder_stable_10]

However, this kernel version has the fix because it’s published in 20180911.

> show version | match kern
JUNOS OS Kernel 64-bit  [20180911.6c98660_builder_stable_10]

Work-around:

1.  If the console is connected with a terminal server, disable  speed auto detection configuration on the terminal server.

2. If the console is connected directly to a laptop, avoid input such as Ctrl+C during the boot up stage, unless prompted.

If the RE still boots up with the gptboot prompt, reboot the RE without the console connection to avoid the interrupt.