Each hard drive can have up to four PRIMARY partitions, any of which could be marked active and bootable. No matter what you might hear, only one of the first four primary partitions can be booted from. That means you can boot from Primary partitions 1, 2, 3 or 4 and that is all. In order to boot openSUSE, you must load openSUSE and the grub boot loader into one of the first four partitions. Or, your second choice is to load the grub boot loader into the MBR (Master Boot Record) at the start of the disk. The MBR can be blank, like a new disk, it can contain a Windows partition booting code or generic booting code to boot the active partition 1, 2, 3, or 4. Or, as stated before, it can contain the grub boot loader. Why load grub into the MBR then? You do this so that you can “boot” openSUSE from a logical partition, numbered 5 or higher, which is not normally possible. In order to have more than four partitions, one of them (and only one can be assigned as extended) must be a extended partition. It is called an Extended Primary Partition, a container partition, it can be any one of the first four and it can contain one or more logical partitions within. Anytime you see partition numbers 5, 6 or higher for instance, they can only occur inside of the one and only Extended Primary partition you could have.
What does openSUSE want as far as partitions? It needs at minimum a SWAP partition and a “/” partition where all of your software is loaded. Further, it is recommended you create a separate /home partition, which makes it easier to upgrade or reload openSUSE without losing all of your settings. So, that is three more partitions you must add to what you have now. What must you do to load and boot openSUSE from an external hard drive? Number one, you must be able to select your external hard drive as the boot drive in your BIOS setup. Number two, you need to make sure that the external hard drive, perhaps /dev/sdb, is listed as the first hard drive in your grub device.map file and listed as drive hd0. I always suggest that you do not load grub into the MBR, but rather into the openSUSE “/” root primary partition which means a primary number of 1, 2, 3 or 4. If number one is used, then that will be out. You will mark the openSUSE partition as active for booting and finally you must load generic booting code into the MBR so that it will boot the openSUSE partition. I suggest a partition setup like this:
- /dev/sdb, Load MBR with generic booting code
- /dev/sdb1, Primary NTFS Partition for Windows
- /dev/sdb2, Primary SWAP (4 GB)
- /dev/sdb3, Primary EXT4 “/” openSUSE Partition Marked Active for booting (80-120 GB)
- /dev/sdb4, Primary EXT4 “/home” Your main home directory (Rest of the disk)
Since I don’t know how much space you want to allocate to openSUSE or what else might be on this hard drive, more info can help provide a better partition setup.
What happens when this external hard drive is not connected? Normally, your BIOS will proceed to the next hard drive in sequence or the next one you have designated to boot from. What happens when you plug in a bootable USB hard drive that has been absent, but was present and booted from at least once? Well this depends really on how the BIOS treats the external device. I have had Dell laptops that do not miss a beat and boot directly from the external hard drive like it has never been gone. I have had Desktops that require you to go back into BIOS setup and reassign the external hard drive as the first boot device again, as if it had never been used. It just depends on how your computer BIOS functions.
Thank You,