Comment 12 for bug 234185

As a first step, I suggest to extend list-devices() with a "usb-partition" option which returns only partitions on USB drives. See https://code.launchpad.net/~tormodvolden/debian-installer-utils/usb-drive-install

Now debian/cdrom-detect.postinst can use this option either 1) all the time or 2) when a special boot option like "usb-media" has been entered or 3) after asking the user if USB drives should be scanned. The alternatives are listed in increasing coding difficulty :) If 1 or 2 are acceptable, I can quickly come up with some patches.