partitioning – I think I screwed up my Ubuntu installation, I cannot copy items onto the desktop and the brightness control doesn’t work

I’m pretty much a Ubuntu newbie.
Okay so I got a new computer and I wiped Win 10 off it and installed Ubuntu 20.04 by USB.

My new computer has 240 GB SSD + 1 TB GB HDD, and I chose to install Ubuntu on that SSD drive. It said that it would fill the drive (but it doesn’t seem to have actually done that? It’s only 8 GB), I thought this was not a problem because then I can just use the other terabyte of HDD for everything else.

Anyway, I installed it (not a dual-boot). I ran into my first problem when trying to switch on the computer after Ubuntu had installed, saying “bootable device not found” but I solved that by opening BIOS settings by holding down F2 and then I went into security options and switched “Legacy” to “UEFI” which had 2 boxes ticked “Ubuntu” and something similar just below it. I also unchecked the options in “Legacy”. After that, my computer would switch on.

So my problem is: I can’t seem to copy anything onto the desktop. Also, the brightness control won’t work. I click on the arrow top right and there’s a brightness slider, and it does nothing.

When I look in System Monitor, it doesn’t show the 1 TB drive, just the 240 GB SSD drive.

Is the way I installed it the problem? Do I have any options to move Ubuntu onto the 1TB or to use that 1TB in my new set-up in any way?

Many thanks for any help