With a kernel built from sources dated 2016-05-29 at 23:57:35 UTC,
when attaching my external hard drive (near-line backup device), I get
the following messages:
uhub0 at usb0: vendor 8086 xHCI Root Hub, class 9/0, rev 1.00/1.00,
addr 0
...
Jun 4 08:21:37 pokey /netbsd: umass0 at uhub0 port 2 configuration 1
interface 0
Jun 4 08:21:37 pokey /netbsd: umass0: Western Digital Ext HDD 1021,
rev 2.00/20.02, addr 24
Jun 4 08:21:37 pokey /netbsd: umass0: using SCSI over Bulk-Only
Jun 4 08:21:37 pokey /netbsd: umass0: failed to create xfers