In general legacy boot does work with LibreELEC installation media.
For getting the "Non-System Disk" error the BIOS seem to perform some additional partition layout or file system checks that fail on the GPT partitioned installer media with protective MBR.
To work around that you can try to set up the hard disk on a different legacy boot machine. The installer will create a MSDOS partition table on disks < 2TB.
Or install a different minimal Linux distribution, create the two LibreELEC partitions and add LibreELEC as default option to the boot manager.
Thank you, I will try and give installing on a different machine a shot.
Do you have any idea if I will run into driver related problems?
For example, if I install on a computer with an AMD graphics card, then throw the hard drive into the HTPC in question, will it be a disaster?
Thanks again.
Up until now, the only info I could find, just said it should work, which wasn't helping me, because it sure doesn't work for me.
I'm so sad that Kodibuntu is no more.
Libreelec is great for Raspberry Pi's and kind, but so far, everything I have tried for full blown PC's has caused nothing but problems since Kodibuntu.