#3-you can install to an external disk. When you get to the window where you have to choose Use Entire Disk, Side by Side or Something Else choose Something Else. Create an ext 4 partition for Ubuntu and a swap partition as well on the external disk. A good rule of thumb is swap = amount of RAM on your machine. If you have 4 GB or more RAM you can get away with no swap, however you will not be able to use hibernate feature.
Once you have the partitions set up (either before install or from the installation program), highlight the ext 4 partition and click Change. Set the mount point to " / ", choose ext 4 as Filesystem, THIS NEXT THING IS VERY IMPORTANT!!!!: there will be a drop down box to choose where the GRUB boot loader is placed. You want to choose the external disk. Assuming you only have one internal hard disk the external should be sdb. DO NOT allow the boot loader to placed on the default sda because then your external will need to be plugged in to boot from the internal disk (Windows). You want GRUB placed on MBR of external disk. Then proceed with the install. Once completed reboot and go into BIOS. Set USB as first device to boot followed by your internal hard disk. This will make it so if your external is plugged in you will get the GRUB boot loader to boot Ubuntu or Windows, if your external is unplugged it will use the Windows boot loader on MBR of your internal disk and boot right to windows.
#1- In Software Sources you can disable the notifications for Release Updates if you wish.
#2- Ubuntu runs well on a lot of machines including those with older hardware. However if you have a problem you should try something lighter such as Lubuntu. Or you can try getting rid of the Unity desktop and install LXDE desktop from Ubuntu Software Center. Once installed reboot. At the sign in screen just above and to the right of the password box click the little ubuntu circular symbol. This will give you option of which desktop you want to use. Choose LXDE and click Use. Your ubuntu will now boot by default into the LXDE desktop session.
When you "uninstall" Linux which was set up as a dual boot with Windows on the same disk you must FIRST restore the Windows bootloader to MBR of your hard disk. If you do not do this GRUB will still be on MBR and it will be pointing to the Linux partition you just removed and you will get a grub rescue prompt. There are two ways to restore the Windows bootloader to MBR BEFORE removing linux. The easiest is to boot into ubuntu. Install lilo either from Ubuntu Software Center or from terminal with this command "sudo apt-get install lilo' (less quotation marks) Ignore warning and continue. Once lilo is installed from terminal run "sudo lilo -M /dev/sda mbr". This will put a generic windows boot loader on the MBR. You can then reboot and you will boot right to windows. You can use Disk Management utility to get rid of former linux partitions. The second method is either before or after removing linux boot from your Windows installation media and restore windows bootloader.
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