To make a flash drive that boots on both legacy BIOS and UEFI machines, use the dd command to write the iso to a flash drive.
First open a terminal and run the command lsblk to determine which device your flash drive is:
If your flash drive is sdc for example, then write the iso to your flash drive as follows. The following command is a single line (even though it might appear to be wrapped into multiple lines in your browser window):
sudo dd if=/home/user/manjaro-xfce-16.06-x86_64.iso of=/dev/sdc bs=4M status=progress oflag=nocache,sync && sync
Wait until the process in the terminal is done. This might take a few minutes.
To make sure the process completes successfully, unmount the flash drive:
sudo umount /dev/sdc
Sometimes the .iso will be written from a buffer to the flash drive on the moment that you unmount the flash drive. That is why unmounting the flash drive can be important. So if that happens when you unmount, then simply wait until the process is done. If nothing happens, then continue.
Finally, open your file manager (Thunar) and eject your flash drive. Now pull your flash drive out of your computer and reinsert it. This is needed so that the Linux kernel can recognize the changes that dd made in the partition table and filesystem of the flashdrive. Rebooting your system will accomplish that as well.
That's it, enjoy.
if stands for 'input file'
of stands for 'output file'
bs stands for 'block size'
status=progress stands for 'show the progress in the terminal'
oflag=nocache,sync stands for 'write the data directly to the flash drive instead of using a cache'
sync makes sure that the iso is fully and correctly written to your flash drive
Warning: Make VERY sure which device your flash drive is (using the lsblk command), otherwise you could accidentally overwrite another device, disk or drive!