Can't boot HP workstation xw6600

Can you boot any thing via USB on that workstation or is it just the LE stick that wont be found...

In the bios make sure to Disable the Secure Boot for Legacy otherwise you may not see the usb option...

If your actually able to boot stuff from USB then as Klojum suggested try a different writing app... You may want to check and make sure that the machine has the last released bios for it installed as there were a few fixes in the last release that fixed a few things, some of which seemed to help booting from a wider variety of sources...

In general tho those machines are not the greatest things to play with things like Kodi as the machine is more designed and suited as a workstation for something like CAD... Last time i ran kodi on one of them was back around the days of Jarvis-Krypton and even then they were kinda buggy and i had to swap out the video card for something more mainstream and then found it didnt play nice with the ECC memory till i swapped it for a different brand...

Not to put a damper on your idea and i am sure with enough playing around you will eventually get it to work but depending on what options are on the machine you may end up playing around with a bunch of things trying to get it to work right... Their were great machines for what they were meant to do but can be touchy trying to get them to run what you want and being a HP makes things a bit worse as like IBM they like to do things their way...

I still have a couple of them here with maxed out cpu's and memory and was using them to run Unity and some PCB Cad software but finally gave up trying to keep up to date versions of software on them as they are just to much work to run newer software on them...