Thank you all so much for your help so far. This is super frustrating (for everyone I'm sure!)
There was no fast/quick boot option, only Quiet boot but I gather that's something different. When I crtl+c and I get into the GRUB interface I get three options: boot into bios, boot into debian, or advanced boot into debian. I've explored these but have not seen any way to access the card so far.
I've taken apart the server again and noticed something, not sure if it's an issue. The SAS controller connects into a Rosewill hot-swap drive cage and the cards connect to the back plane with the SAS interface. But on the cards, next to this interface, there are four pins labelled MOLEX 78326 which are not connected (see attached). Is that fine? I guess that they are just an alternate power source?
And thanks again. I get that I'm asking a lot of hardware questions on a software forum.