CAM_REQ_CMP_ERR

[grarpamp]

I get a lot of these. Searching doesn't say much about them.
What do they mean?
A data corruption error on the media?
 (Seems to happen at random (nonrepeating) locations, so I doubt this.)
Is sysctl kern.cam.da.retry_count the correct way to bump this retry count?
 (I tried that and it seems to bump, but doesn't seem to have much
  result as far as turning the error into a later success.)
Can I force various interface speeds like USB 1.0, 2.0, etc?
Thanks.

cam.h: CAM_REQ_CMP_ERR, /* CCB request completed with an error */
8 STABLE r269631

kernel: (da0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
kernel: (da0:umass-sim0:0:0:0): Retrying command
...
kernel: (da0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
kernel: (da0:umass-sim0:0:0:0): Error 5, Retries exhausted

ugen3.2: <MAKE> at usbus3
umass0: <MAKE MODEL, class 0/0, rev 2.00/0.00, addr 2> on usbus3
umass0:  SCSI over Bulk-Only; quirks = 0x4100
umass0:3:0:-1: Attached to scbus3
(probe0:umass-sim0:0:0:0): Down reving Protocol Version from 2 to 0?
pass2 at umass-sim0 bus 0 scbus3 target 0 lun 0
pass2: <MAKE MODEL 2.00> Removable Direct Access SCSI-0 device
pass2: Serial Number num
pass2: 40.000MB/s transfers
GEOM: new disk da0
da0 at umass-sim0 bus 0 scbus3 target 0 lun 0
da0: <MAKE MODEL 2.00> Removable Direct Access SCSI-0 device
da0: Serial Number num
da0: 40.000MB/s transfers
da0: sizeMB (count 512 byte sectors: 64H 32S/T sizeC)
da0: quirks=0x2<NO_6_BYTE>
da0: Delete methods: <NONE(*)>