Comment 81 for bug 466575

The same problem on Ubuntu Precise:

sudo udisks --detach /dev/sdc
Detach failed: Error detaching: helper exited with exit code 1: Detaching device /dev/sdc
USB device: /sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.7)
SYNCHRONIZE CACHE: FAILED: No such file or directory
(Continuing despite SYNCHRONIZE CACHE failure.)
STOP UNIT: FAILED: No such file or directory

This error message matches exactly the the error message from Nautilus.

Apparently, Unity fails to safely remove the drive too.
Though it does not report a problem, it does not power down the device, and it is still available in Nautilus for mounting.
Truly removed devices disappear from the list of mountable devices until they are unplugged and plugged again.

On my computer USB thumb drives usually do not produce this error.
But USB HDD always produce this error.

My guess is that HDD contain more than one entry in the partition table, while thumb drives usually have only one, and this somehow matters.