I have the same problem with legacy boot from USB drive on a DL G10 server.
I need a solution from HPE.
BIOS is configured to legacy boot.
USB boot is activated, no setting that would prevent booting from USB.
(It's not UEFI boot, so ther is no secure boot, uefi optimized boot or anything)
The USB drive boots on every system I tested, including DL G9 server.
Why is it so hard for HPE to get legacy boot working in G10 servers????
There is no alternative for me.