Buffer I/O Errors with emulated usb disk image

Binary package hint: qemu-kvm

Description: Ubuntu 10.04.1 LTS
Release: 10.04

qemu-kvm version 0.12.3+noroms-0ubuntu9.2

I expected using disk images with the -usbdevice option to work without errors.

Instead I get lots of I/O errors on the guest operating system.

When trying to use a emulated usb disk from an image like so:

   qemu -hda root.img -usbdevice disk:usb.img

When I write to the simulated disk in the guest operating sytem (Debian or Ubuntu)
I get several errors like this:

"Buffer I/O error on device sda1, logical block 570" (dozens)
"end_request: I/O error, dev sda, sector 656"

This also generally causes the disk image to become corrupted.

I have tried this with various image formats (including raw and qcow2)
and with both FAT and ext4 filesystems. I have tried creating the file systems
inside and outside of qemu.

The same disk images work fine if I attached them to a loop device
then give the loop device as the argument to -usbdevice:

    sudo losetup /dev/loop0 usb.img
    qemu -hda root.img -usbdevice disk:/dev/loop0