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How do you create a bootable partition on a USB drive?
I have a 250G external hdd, I would like to make it bootable to install Ubuntu along side win7, but how can I make my hdd a bootable thing. What I did is to use diskpart utility and copy Ubuntu iso files into hdd, and change bios boot order to boot from usb external storage first, but it always says missing mgr... what should I do?
If you first set the partition to active, then copy the ISO to the disk (assuming you mean extracting the files), this will only boot in to the Ubuntu Live/Installer environment (If they haven't changed it).
You will also need to choose the boot device at startup.
Instead, what I recommend you try is the Linux Universal USB Installer.
If however you want to actually install Ubuntu full to the external drive (not just live), as the boot order can change and sometimes you may not have the external drive, I would recommend disconnecting your primary drive, or choosing disabled in the BIOS, then installing it normally to the external drive.
When you want to run Linux, just hit F10 or whatever it is on your machine, choose USB and you should be good to go!
Take a look at Unetbootin, it was designed exactly for this and works with a myriad of distributions.
UNetbootin can create a bootable Live USB drive, or it can make a "frugal install" on your local hard disk if you don't have a USB drive. It loads distributions either by downloading a ISO (CD image) files for you, or by using an ISO file you've already downloaded.
Copying the ISO images to a HD will not work. ISOs need to be burned to a CD or DVD (Windows 7 does it natively - right click and choose burn disc.) Then boot off the DVD and through the install select your external HD. The setup will guide you through partitioning it and it will make it bootable. There maybe be a way to burn the ISO to the HD by making it think its a DVD drive, but that'd be way more complicated than just burning a DVD.
If you have a CD/DVD drive to boot the Ubuntu CD/DVD, then the installation of the OS on your external drive should also make it bootable. This includes marking the newly created Ubuntu partition as bootable and installing GRUB bootloader on it at the end of the setup.
After this, you should be able to boot it by choosing to boot from USB on the system startup.
If you don't have a CD/DVD drive, see John T's answer on how you can make a separate USB drive/stick bootable in order to install the OS. (Booting the Ubuntu installer from the same drive you are installing on may be possible, but not very easy.)
