I would suggest you imitate whats going on in regards to dd'ing the .img to USB
Meaning; mount the .img file and copy its contents to your USB in lieu of actually doing the dd process
I dont prefer to dd my usb just to test a distro, so I mount the .img and copy it to a folder(to check it out)
or directly to USB.
I'm assuming Chromium OS is a Debian based system or no??
For example; earlier I downloaded UNR (ubuntu-netbook-remix), which is a .img file made to be dd'd
Because I often test distro's from my 2 usb's, I dont want to dd anything
as I will have to reformat it later
So, I mount the .img file and copy to folder or usb:
cd into the folder wherever the .img is
assuming its on Desktop, and folder name is "newusb"
become root too:
for me, UNR.img is in the newusb folder
so gotta make a folder to mount the .img on, any name is good
I'll use "mnt"
and a folder to copy the contents of .img file to
I'll name that folder "copy"
Now mount the .img file on "mnt" folder
and copy to "copy" folder
mount -o loop ubuntu-9.04-netbook-remix-i386.img mnt
there may be a hidden file/folder in there too so make sure to copy any of those too
Now, unmount the folder mnt and see whats in the "copy" folder
Your USB needs to be reformatted
and fixed before you do anything