I've installed it on a usb flash drive, before I was using FreeNAS,now wanted to switch to ubuntu server,but it doesn't boot. I've checked my Bios to see if there's anything wrong with the booting options but everything looks just fine.
This seems to be a common question around forums.
A universal solution (Linux/Windows/OSX) for creating bootable flash media is UNetbootin. I've read that it works with Windows isos as well but if you run in to problems using a Windows OS iso (and your in Windows) then try the Windows 7 USB/DVD download tool
You'll need a large enough flash drive and the a copy of iso that you want. Windows 7 USB/DVD tool (and UNetbootin as far as I'm aware) will automatically blank and format the drive.
Having successfully created the bootable drive, you then need to tell the BIOS to boot from it. This may involve going into the BIOS menu on boot up and set manually, or there may be keypress (Esc) shortcut to choose the boot device. This should be apparent on startup but if you can't get into the BIOS or boot menu, do google search or consult your motherboard/laptop manufacturer's support page.