I know that in general this means that I have a "bad disk". But I'm after a more specific reason for why I am getting these messages from the kernel:
sd 15:0:0:0: [sda] Attached SCSI disk sd 15:0:0:0: [sda] Unhandled sense code sd 15:0:0:0: [sda] Result: hostbyte=0x10 driverbyte=0x08 sd 15:0:0:0: [sda] Sense Key : 0x3 [current] sd 15:0:0:0: [sda] ASC=0x11 ASCQ=0xc sd 15:0:0:0: [sda] CDB: cdb=0x28: 28 00 00 00 00 00 00 00 20 00 end_request: critical target error, dev sda, sector 0
I've had a bit of a search and I see that 0x3 sense key means "Medium error" and ASC=0x11 means "Read error". But it is still a mystery as to what the ASCQ=0xc means.
The device is a bus powered USB drive which reports its model as "TOSHIBA MQ01ABB200"