can't boot from usb

I created a boot usb for 18.3 using the dd command. The command appeared to work with no problem: 3710016+0 records in, same number out 1.9 GB copied. When I try to boot from it I get the reply that there is no operating system. I checked the contents against my old 17.3 flash usb which I'm still able to boot from. They are similar - there is however one discrepancy: vesamenu.c32. 17.3 says it's a binary file 153.0kB, 18.3 says the file type is unknown 26.7kB. Is this the problem, should I redo the dd on a clean usb, can I use a usb that i cleared all files from???