I have this error message too on Ubuntu 10.04 and 10.10, amd64. This is an external USB3.0 2,5" hard disk on an USB2.0 port (my laptop and my desktop don't have USB3.0 ports yet, but the external hdd came with one).

Error message:
Error detaching: helper exited with exit code 1: Detaching device /dev/sdb
USB device: /sys/devices/pci0000:00/0000:00:1d.7/usb2/2-2)
SYNCHRONIZE CACHE: FAILED: No such file or directory
(Continuing despite SYNCHRONIZE CACHE failure.)
STOP UNIT: FAILED: No such file or directory

Umounting from the command line produces no error:
$ sudo umount /media/ELSTER
$

The file system is ok:
$ sudo fsck -fv /dev/sdb1 
fsck from util-linux-ng 2.17.2
e2fsck 1.41.12 (17-May-2010)
Durchgang 1: Prüfe Inodes, Blocks, und Größen
Durchgang 2: Prüfe Verzeichnis Struktur
Durchgang 3: Prüfe Verzeichnis Verknüpfungen
Durchgang 4: Überprüfe die Referenzzähler
Durchgang 5: Überprüfe Gruppe Zusammenfassung

      12 inodes used (0.00%)
       0 non-contiguous files (0.0%)
       0 non-contiguous directories (0.0%)
         # von Inodes mit ind/dind/tind Blöcken: 0/0/0
         Erweiterungstiefe Histogramm: 2
  352958 blocks used (1.81%)
       0 bad blocks
       1 large file

       0 regular files
       3 directories
       0 character device files
       0 block device files
       0 fifos
       0 links
       0 symbolic links (0 fast symbolic links)
       0 sockets
--------
       3 files

This is the usb device:
$ lsusb -v -d 174c:5106
Bus 002 Device 008: ID 174c:5106 ASMedia Technology Inc. 
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x174c ASMedia Technology Inc.
  idProduct          0x5106 
  bcdDevice            0.01
  iManufacturer           2 ASMedia
  iProduct                3 AS2105
  iSerial                 1            77AWT06ET
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           32
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xc0
      Self Powered
    MaxPower                0mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         8 Mass Storage
      bInterfaceSubClass      6 SCSI
      bInterfaceProtocol     80 Bulk (Zip)
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
Device Qualifier (for other device speed):
  bLength                10
  bDescriptorType         6
  bcdUSB               2.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  bNumConfigurations      1
Device Status:     0x0001
  Self Powered