Notebook Boot and Lockup

Hi everyone. I'm having problems selecting the M.2 SSD drive I installed as the primary boot drive in legacy mode in BIOS: When I enter the hard drive boot order list in the legacy boot order window it either:

a) Presents me one drive ("Notebook Hard Drive - ST1000LM014...") and <null string> or <gibberish>, or

b) Goes berzerk for a while, presenting me a rolling screen in two shades of blue, followed by random psychedelic floating symbols, before it returns to a faulty version of the BIOS setup screen (missing items, labels, screen parts etc.)

In the former case I can move the SATA drive down the list with F5 in the list, making the M.2 drive appear in its place, but the setting doesn't persist or register - as soon as I re-enter the boot order list the SATA drive reappears at the top (usually with <null string> below it). Moving the SATA drive down into the lower of the two positions is the only way to even make the M.2 drive appear in the list.

There is no problem booting from the drive with a UEFI OS installed: The drive appears correctly and consistenly in the UEFI boot order list under OS boot manager, and boots as expected.

The M.2 SSD I installed is a Crucial CT275MX300SSD4. The laptop's BIOS and SSD firmware are up to date. If anyone has a suggestion as to how to make this the primary legacy boot drive, I'm listening.

Cheers.

Edit: The psychedelic symbols turned out to be Chinese characters 😉