My RB450 won't boot.
While I have found a lot of information online, nothing I have found seems to match my circumstances.
I was given this old RB450. It was working fine, running RouterOS 4.9. The Webfig interface was very different to the newer models I am familiar with - it had a minimal horizontal menu structure.
So I downloaded an update - went for an older version, thinking it might be better with my older hardware - routeros-mipsbe-6.21.npk
The file uploaded and I clicked the install (or similar) button. Since then, the RB450 has not booted.
I've got a serial console going. This is the output when I attempt to boot:

RouterBOOT backup booter 3.02

RouterBoard 450

CPU frequency: 300 MHz
Memory size: 64 MiB
NAND size: 128 MiB

Press any key within 2 seconds to enter setup..

loading kernel from nand... OK
setting up elf image... OK
jumping to kernel code
ERROR: no system package found!
Kernel panic - not syncing: Attempted to kill init!
Rebooting in 1 seconds..

Details of the board are:

your choice: i - board info

Board Info:

Board type: 450
Serial number: 36E002C01507
Firmware version: 3.02
CPU frequency: 300 MHz
Memory size: 64 MB
Build time: 2012-11-30 17:04:49
eth1 MAC address: D4:CA:6D:9C:B8:1E
eth2 MAC address: D4:CA:6D:9C:B8:1F
eth3 MAC address: D4:CA:6D:9C:B8:20
eth4 MAC address: D4:CA:6D:9C:B8:21
eth5 MAC address: D4:CA:6D:9C:B8:22

press any key to continue...

I wondered if I need to update my bootloader to match the RouterOS that I uploaded? Uploading it would be OK, if I could find a file. I've not found anywhere to download an updated bootloader.
Obvious option would be downgrade the RouterOS back to 4.9, but I can't see a way to do that in the serial interface.

Can someone please ether give me some steps to get my RB450 out of this pickle, or point me to instructions that clearly show the steps and how to find the files for this model?

Cheers! Steve

Fixed it myself! Documenting here for the benefit of others.
This document describes use of the serial cable. https://wiki.mikrotik.com/wiki/Manual:RouterBOOT Sorry - this will be a steep learning curve if it is new to you. I date back to the days of analouge modems, and use various other routers via serial cable when I can't avoid it.
I used the serial cable to force booting off ethernet. If the machine tried to boot from NAND, it would crash. If boot simply failed, then it could be set to use ethernet as on fail, but the boot did not fail elegantly, the router just crashed, so it never tried ethernet. Ethernet had to be first option.
Netinstall seemed to be a mystery. I was following these instructions, but it was not working: https://wiki.mikrotik.com/wiki/Manual:Netinstall
Then I looked more carefully at my laptop, and the instructions, and disabled VMware and pCap related interfaces, as well as the more obvious ones. With all interfaces disabled, i was able to get NetInstall working, following the instructions above. One other caveat - it always seemed to take two attempts - first boot attempt failed. Leave the router to try again (about 3 minutes), and serial port announced it had worked. I did other fluffing about with NetInstall, not sure how much was necessary.
At this point I felt I had control of my router. I tried 6.21 again. Failed with Kernel panic as before. I then coorced NetInstall to work again, and tried routeros-mipsbe-6.42.12.npk. That installed, and after changing the boot device back to NAND, router booted with the very latest RouterOS! (Many years newer than 4.6 that I started with!) Mikrotik Rocks!