Comment 4 for bug 253599

It really depends on what the usb "disk" is. For usb memory devices (not card readers), the appropriate naming scheme is media-flash-usb-$make-$model. For usb memory devices that are memory cards, the mounted icon names should be media-flash-$type-$make-$model (or simply media-flash-$type). And for usb devices that are actually hard disks, the drive-harddisk icon should be used (one could add the -usb classifier, but it's generally not important information for external hard disks that are usb. Sticking a usb icon on a harddisk isn't particularly useful information in the icon, from a usability perspective.

So really, this bug (and others like it) should probably be filed against whatever package is providing the drive-harddisk-usb icon name, for usb storage devices that aren't hard disks.