The 9455XL is fitted with a standard Award BIOS that has been slightly customised by Wyse. The BIOS is entered by hitting the DEL key during the power-up sequence. If you find the BIOS is password protected (mine was) the default password is Fireport.
The BIOS boot options are:
Of course, if you have no interest in the 'WINTERM' aspect of the device, there is no reason why you
can't dump this BIOS and install the standard VIA BIOS available from the VIA website.
https://www.viatech.com/en/support/eol/epia-eol/
I have had no joy trying to boot pen drives using the USB-HDD option. However this article describes how you can set up a pen drive that will boot using the USB-ZIP option.
Note: Frank in Holland has discovered that the BIOS on this board (in LBA mode) only supports disks up to 137GB. A quick Google showed various (inconclusive) discussions on this topic. He emailed VIA support and their response was essentially:
"The maximum disk size is 137GB. The board is ancient and no longer supported. You should buy something new."
The 9455XL runs Windows XP embedded according to the licence label on the back. My came with the flash wiped clean.