This may be because vfat has a file size limitation of 4 GB. That is why it worked on your smaller 4 GB flash drive but failed on your larger drive when you tried to make the 7 GB file. Check the casper folder on your usb drive and see if there is a casper-rw file. If not then that is your problem. It wasn't created by usb-creator. Create the persistent usb again. Then look at ~/.usb-creator.log. My logs show an invalid size error from dd, and thus the casper-rw file isn't created.
This may be because vfat has a file size limitation of 4 GB. That is why it worked on your smaller 4 GB flash drive but failed on your larger drive when you tried to make the 7 GB file. Check the casper folder on your usb drive and see if there is a casper-rw file. If not then that is your problem. It wasn't created by usb-creator. Create the persistent usb again. Then look at ~/.usb-creator.log. My logs show an invalid size error from dd, and thus the casper-rw file isn't created.
This is related to: https:/ /bugs.launchpad .net/ubuntu/ +source/ usb-creator/ +bug/284085