Zfs on usb-disk checksum errors?

[Ronald Klop]

Hi.

I put zfs on my external usb-disk, so I can backup my harddisk with zfs  
send/receive.
I now have corruption on this volume.

[root at sjakie ~]# zpool status -v
   pool: extern
  state: ONLINE
status: One or more devices has experienced an error resulting in data
         corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
         entire pool from backup.
    see: http://www.sun.com/msg/ZFS-8000-8A
  scrub: scrub completed after 0h2m with 0 errors on Wed Jul  8 00:35:09  
2009
config:

         NAME        STATE     READ WRITE CKSUM
         extern      ONLINE       1     0     0
           da0       ONLINE       9     0     0

errors: Permanent errors have been detected in the following files:

         <0x3f>:<0xf5d6>

I don't really understand which files have corruption. :-(
In my syslog is this: (repeated quite often)
Jul  8 10:00:37 sjakie kernel: (da0:umass-sim0:0:0:0): SYNCHRONIZE  
CACHE(10). CDB: 35 0 0 0 0 0 0 0 0 0
Jul  8 10:00:37 sjakie kernel: (da0:umass-sim0:0:0:0): CAM Status: SCSI  
Status Error
Jul  8 10:00:37 sjakie kernel: (da0:umass-sim0:0:0:0): SCSI Status: Check  
Condition
Jul  8 10:00:37 sjakie kernel: (da0:umass-sim0:0:0:0): ILLEGAL REQUEST  
asc:20,0
Jul  8 10:00:37 sjakie kernel: (da0:umass-sim0:0:0:0): Invalid command  
operation code
Jul  8 10:00:37 sjakie kernel: (da0:umass-sim0:0:0:0): Unretryable error

and sometimes
Jul  8 10:00:35 sjakie root: ZFS: vdev I/O failure, zpool=extern  
path=/dev/da0 offset=127558877184 size=3072 error=5
Jul  8 10:00:35 sjakie root: ZFS: vdev I/O failure, zpool=extern  
path=/dev/da0 offset=127558877184 size=3072 error=5
Jul  8 10:00:35 sjakie root: ZFS: zpool I/O failure, zpool=extern error=5
With varying offsets and sizes.

What can I conclude from this? Is the disk failing? Is the 'Invalid  
command operation code' something to worry about? It didn't show up when  
the disk was UFS.

I reinstalled the pool but the read-errors showed up again.

Thanks for any advice,

Ronald.