Hi Reload,
I believe this is where you are getting stuck at:
SSG550-> save software from tftp 172.22.152.251 new/ssg500.6.3.0r17.0 to flash
Load software from TFTP 172.22.152.251 (file: new/ssg500.6.3.0r17.0).
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
(snip)
tftp received octets = 11627247
tftp success!
TFTP Succeeded
Save to flash. It may take a few minutes ...platform = 23, cpu = 11, version = 2
update new flash image (02572fd0,11627247)
platform = 23, cpu = 11, version = 2
offset = 20, address = 0, size = 11627169
date = 9422, sw_version = 808031, cksum = 954806c3
********Invalid image!!! ********Bogus image - not authenticated!!!
(snip)
If you would like to update the boot loader that is signed with the new image key on SSG Series, you must have a console connection and a TFTP server that can be reachable through the pre-assigned interface(s) in the boot loader mode (mostly ‘eth0/0’ interface) and manually interrupt the boot sequence by holding ‘Shift key’ and hit 'X' and ‘A’ sequentially when the “Hit 'X' and 'A' to upgrade bootloader” message is shown on the console.
After installing the new image key, type CLI reset command to reboot the device. Then keep the ‘Shift key’ down and hit 'X' and ‘A’ sequentially.
SSG550-> reset System reset, are you sure? y/[n] y In reset ... (snip) ScreenOS Saipanloader V1.0.7 Built Mar 19 2009/15:54:12 watchdog_probe, 1132 bus/dev/fn = 0/248 ich = 2640 boot_drive = 80 start1 = 0768, start2 = 3840 Hit 'X' and 'A' to upgrade bootloader<- Hold ‘Shift key’ and hit ‘X’ and ‘A’ in sequenceLoader File Name:new/Loadssg500v107.d <- Bootloader filename signed with the new image keySelf IP Address :172.22.152.35 <- TFTP client IP addressTFTP IP Address :172.22.152.251 <- TFTP server IP addressIP MASK :255.255.255.0Gateway IP Address :172.22.152.1 Saipan motherboard proto 3 or later detected Probing...[Ethernet0/0 and Ethernet0/1] Initiating hardware and waiting for link up ... Initiating hardware and waiting for link up ... self_ip = 172.22.152.35, tftp_server_ip = 172.22.152.251 ip = 172.22.152.35 mask = 255.255.255.0 gw = 172.22.152.1 svr = 172.22.152.251 network_ready = 1 new/Loadssg500v107.d 121078 bytes downloaded from tftp server old img size = 121032, new img size = 121032, load = 121078, sig = 46 S Image authenticated!<- Bootloader is authenticated using the new image key
<snip>
write boot2's start sector back at sector 1051 write mbr back at sector 0 mounting FAT16 partition file size = 112 size = 112, sizeof(nvram_rec) = 112system rebooting...<- After successful bootloader installation, the device will automatically try to reboot
<snip>
The below document should help you:
https://www.juniper.net/documentation/software/screenos/screenos6.3.0/630_upgrade.pdf
Hope this helps 
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