I have tried to boot my netbook from a USB drive and have found that the netbook wouldn't boot to the USB Drive if the partition was too large.
I had success with a 128 MB (yes I do mean megabyte) and a 512MB that was partitioned to show only 100MB. The 512MB would not boot if the whole 512MB was left as one partition.
This situation may be particular to my netbook or its BIOS, but it is something to be considered. The link below has quite a bit more information on this subject.