Mavericks needs 8 GB space.
Not sure your Lexar 8GB drive has that space free.
Check for it.
I use following process to make a bootable USB.
Format it, using Disk Utility, as a Mac OS Extended (Journaled) drive, named Mavericks.
This can be done from the Erase tab within the app; make sure the USB drive does not have multiple partitions (that can happen, so turn to the Partition tab to verify and correct this).
Open Terminal and Copy/Paste following.
sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/Mavericks --applicationpath /Applications/Install\ OS\ X\ Mavericks.app --no interaction.
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk…(this part can take 30 minutes)
Making disk bootable...
Copying boot files...