I know this question sort of has been answered elsewhere, notably, here:
except that i am having an error that i do not know what to do with.
I am getting a "Failed to unmount partitions" on "/cdrom" right after partitioning the usb hard drive.
I've Googled this error without finding an accurate match, i have found errors related to CD-ROM on server editions of ubuntu except that the fix doesn't apply to me (no files to rename and path is different)
I have to admit I am not savvy enough with ubuntu to know how to troubleshoot this error.