Slept on it
OK, you can run this from a terminal with your USB or any external/inetrnal and see how the sytem treats it, this is my VM with two USBs plugged in, important is the the third column displayed in the default lsblk output is RM: by looking at this column we can see if the device is removable (the value would be 1), or 0 for not. In the output below, you can see the USBs I have plugged in, all are marked as removable, sr0 is an optical drive with the sme10 ISO.
[root@sme10min ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 15G 0 disk
├─sda1 8:1 0 500M 0 part /boot
└─sda2 8:2 0 14.5G 0 part
├─main-root 253:0 0 13G 0 lvm /
└─main-swap 253:1 0 1.5G 0 lvm [SWAP]
sdb 8:16 1 7.2G 0 disk
└─sdb1 8:17 1 7.2G 0 part
sdc 8:32 1 3.8G 0 disk
└─sdc1 8:33 1 3.8G 0 part
sr0 11:0 1 1.5G 0 rom
The console backup routines use this info