Often dd suffices.
For instance I use this:
dd bs=4M if=$DEPOT_ISO/zenwalk-current-15.0-200703a.iso of=/dev/sdc conv=fdatasync status=progress
Where $DEPOT_ISO is the environment variable for where I keep all .iso files.
/dev/sdc being my usb device. Always check that you use the correct /dev/ entry, via fdisk -l,
df or whatever the command was (I use aliases so I don't remember the commands, for instance
"usb?" will tell me on the commandline which USB devices are mounted and so forth).
There is also the option to put multiple .iso files onto the same USB device and get an option.
I forgot the name but you can google it; many computers no longer come with optical drives
so I had to transition to this. I actually accidentally bought a computer case without
an optical drive.
Using USB is quite nifty though and the multi-USB .iso way is recommended. Even alienbob
from slackware recommends it these days. So this is the future, IMO!
Having it on USB 2 or USB 3 should not make a huge difference except that USB 3 may
be faster and probably have more current available. But I never really had a problem with
USB 2, and I don't pay attention while things get installed onto the hd anyway. I resume
working on that computer after it has finished. (I use more than one computer, to avoid
downtime problems and what not.)