Just to make sure, you have 2 USB disks: first with Qubes OS installer and second where you want to install Qubes OS on?
Switch to the shell on another TTY by pressing Ctrl+Alt+F2 and check the output of these commands to see if your USB disk is detected by the system:
lsbsk
lsusb
You can also try to check the log messages related to USB to see if there are any errors:
journalctl | grep -i usb | less
You can use PgUp/PgDown to scroll the log.