I've attached a SATA disk using a SATA-to-USB3 adapter. dmesg output said it shows up as "/dev/sdh", but gparted didn't show it - and gparted threw the following error message:
[SOLUTION]Problem opening /dev/sdh for reading! Error is 123.
It was a 3.5" regular spinning HDD, and I was using a bus-powered SATA-to-USB adapter. So the disk didn't even manage to spin up. I had to try it...
So I've used an externally powered docking station, and everything was fine