did not pick up

In what sense?

Like, if you do something like:

$ lsusb > not-in.txt

Then plug it in (I'd have the SD card in, but I don't think that it'd likely matter at the USB level) and do:

$ lsusb > in.txt

And then do:

$ diff -u not-in.txt in.txt

No differences show up?

If the host can talk to the USB drive, then I'd expect it to show up there.

EDIT: If it shows up there, then I'd think that the machine is talking to the device at a USB level. To see whether the host can see the block device corresponding to the SD card, I'd do the same with lsblk (for this one, the SD card probably has to be in the reader):

$ lsblk > not-in-blk.txt

Insert the USB reader into the machine.

$ lsblk > in-blk.txt

And then:

$ diff -u not-in-blk.txt in-blk.txt

If it can see that, then it can see the block device with the data on it, the card. The filesystem or something like that might be corrupt or something, but should still show up at that level.

EDIT2: If the above works and you're wanting KDE Plasma to pop up a window when you plug the thing or something and that's not happening and that's what's you're concerned about that...I don't use KDE Plasma, so I've no idea what expected behavior is there. Maybe someone who uses it could tell you. I imagine that it has some kind of filesystem browser and it's probably visible somewhere there, but...shrugs

In a plain-Jane Linux sense, if you have a block device, you'll probably have one block device, and maybe it'll be partitioned and have smaller block devices for each partition. Maybe /dev/sdb for the main device or something and /dev/sdb1 for the partition. If you can see the block device for the partition (like, you can see it appear and disappear when running lsblk when you plug in and unplug the device), you can mount the block device in the traditional Linux way (with the correct block device where I'm using /dev/sdb1 below):

$ sudo mkdir /mnt/sdcard
$ sudo mount /dev/sdb1 /mnt/sdcard

And then before you unplug it:

$ sudo umount /mnt/sdcard

That'll be independent of any GUI filebrowser or whatever.