The Disks utility doesn't allow formatting the disk. I, also, tried running sudo fdisk /dev/sda, but it returned cannot open /dev/sda: No medium found.
I tried it with two different adapters, two different SD cards and two different USB ports, so the problem is not likely to be with those devices.
Interestingly, one SD card was recognized at first, but, after I tried flashing some .iso to it, it immediately vanished from the view of my machine.
lsblk lists sda only when run with the -a flag, which means that the sda is "an empty device" as per man lsblk. I don't really know how to interpret that.
Here's the full output of sudo lsblk -a:
sda 8:0 1 0 disk
nvme0n1 259:0 0 477G 0 disk
├─nvme0n1p1 259:1 0 512M 0 part /boot/efi
└─nvme0n1p4 259:2 0 476,4G 0 part /
P.S. Sorry for not embedding the images correctly. Due to some server problems, I wasn't able to do that.
/dev/sda? Not/dev/sdborcor further? What doeslsblkgive?lsblkreturns/dev/sda. I've added the full output oflsblkto the post