Cannot create bootable disk with USB drive

Hello,

I've been trying to create a bootable disk of Ubuntu on a SanDisk USB drive to install on another computer, and I have been having difficulties creating it. When I try to do it by the "restore" method in Disk Utility, I can type in my admin. After I type that in, the Destination disappears, and depending on if I have the partition selected or the .dmg / .iso file (I've tried both) selected it does different things. If I have the file selected, the destination partition disappears and it tells me it needs to scan the disk and asks me for my password. When it is given, it gives me an error: "Unable to scan 'Example.dmg'. (Invalid argument)". If the partition is selected, the destionation still disappears, but it replaces the .dmg file with the partition, and asks to scan once again, but after the formalities, it gives me this error: "Unable to scan 'Example-partition'. (Resource Busy)" (and I don't have the drive selected, I have the partition selected. If I try to do it in terminal, I cannot remount the disk after the software is put on it, and it doesn't show up in the option mode menu. Yes, I have mad the disk Mac os extended (Journaled). This is being done on a 2014 Macbook pro with the latest version of OS X (10.10.2) installed. Any suggestions or information would be much appreciated, Thank you very much for your help.