For my money I always do a
for Debian/Ubuntu distros replace "su" with "sudo su".
This will display all the disks and their partitions.
From the size of the disk I can find out if it is a USB device and its device names directly from the kernel.
A device, if has a vaild filing system, say it is sdc1 can be mounted manually by commands
mount /dev/sdc1 /mnt/sdc1
The last command list the contents of sdc1. By such time the folder /mnt/sdc1 of the device will be displayable in the GUI to do drag and drop.