Also, if trying recovery, then from the “Select a recovery image”/GNU, if we select the GRUB and follow the Junos recovery procedure with format Junos data disk and format Junos config disk, that should work.
To answer my own question, and in case this information is useful to anyone else:
The USB ports appear to be 2.0:
usb1: EHCI version 1.0 usb1: <EHCI (generic) USB 2.0 controller> on ehci0 usb1: USB revision 2.0
Data transfer speeds to a 3.0 device overall showed higher throughput than to a 2.0 device, but not dramatically so. Regardless, total time to restore from a USB install image was identical whether using 2.0 or 3.0: 7 minutes 40 seconds.