USB SD-card reader recognized, but not working, on 6.1

[perryh at pluto.rain.com]

When I plug in the reader, I get (on the console):

umass0: SDMMC M121 USB 2.0  SD/MMC READER, rev 2.00/2.01, addr 2
da0 at umass-sim0 bus 0 target 0 lun 0
da0: <USB 2.0 SD/MMC Reader \001\000\000?> Removable Direct Access SCSI-0 device 
da0: 1.000MB/s transfers
da0: 1962MB (4019200 512 byte sectors: 255H 63S/T 250C)

So far, so good, and it appears -- based on this excerpt from the
mtoolstest output -- that mtools is configured to read /dev/da0 as b:

drive B:
        #fn=1 mode=0 defined in /usr/local/etc/mtools.conf
        file="/dev/da0" fat_bits=0 
        tracks=0 heads=0 sectors=0 hidden=0
        offset=0x0
        partition=0

However, when I try to read it with mtools:

$ mdir -a b:
init B: non DOS media
Cannot initialize 'B:'

When I try to investigate using file:

$ file -s /dev/da0

I get a very long pause, during which this appears on the console:

umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT
umass0: BBB reset failed, TIMEOUT
umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT
umass0: BBB reset failed, TIMEOUT
umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT
umass0: BBB reset failed, TIMEOUT
umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT
umass0: BBB reset failed, TIMEOUT
umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT
umass0: BBB reset failed, TIMEOUT
umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT
(da0:umass-sim0:0:0:0): Synchronize cache failed, status == 0x4, scsi status == 0x0
umass0: BBB reset failed, TIMEOUT
umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT

and the file command reports

/dev/da0: ERROR: cannot read `/dev/da0' (Input/output error)

Is there anything that can be done in the way of configuration
adjustments, or is this reader just not usable on 6.1?

USB part of dmesg.boot:

uhci0: <Intel 82371AB/EB (PIIX4) USB controller> port 0xcce0-0xccff irq 11 at device 7.2 on pci0
uhci0: [GIANT-LOCKED]
usb0: <Intel 82371AB/EB (PIIX4) USB controller> on uhci0
usb0: USB revision 1.0
uhub0: Intel UHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub0: 2 ports with 2 removable, self powered