Post about booting multiple operating systems from USB.

know-how/software/_posts/2016-03-11-multi-usb-boot.md

@@ -0,0 +1,155 @@
+---
+title: Multi USB Boot
+layout: default
+created: 2016-03-11 19:36:55 +0100
+updated: 2016-03-11 19:36:55 +0100
+toc: false
+tags:
+  - know-how
+  - software
+  - usb
+  - flash-drive
+  - booting
+---
+Similar to [booting different operating systems via network]({% post_url 2016-03-05-ipxe-network-boot %}),
+you can also boot multiple systems from a USB flash drive.
+
+![]({{ site.url }}/assets/multi-boot-grub.jpg){: width="400px"}
+
+This is even a bit easier since you have a boot medium (the flash drive) and
+don't have to jump through hoops to get a system to boot from the network.
+
+
+Initial Setup
+=============
+
+While you *can* use the whole flash drive for everything, I found it more neat
+to have separate partitions for each tool. So I started out by creating a 16 GiB
+partition on my 32 GiB flash drive. This serves as the main partition to boot
+from. Make it a primary partition and format it with ext4[^1].
+
+Now, we need to install the bootloader to it. I chose `grub`, others use
+`syslinux`. Both are suitable, so it's up to you. I followed [these](https://www.pendrivelinux.com/boot-multiple-iso-from-usb-via-grub2-using-linux/)
+instructions. (Omitting the `wget` command because I wanted to have my own
+`grub.cfg`.)
+
+First, mount the new partition so the neccessary files can be copied to it. My
+partition is `/dev/sdc1`, so I mount it with:
+
+    sudo mount /dev/sdc1 /mnt/usb
+
+Now, to install `grub`, just do a:
+
+    grub-install --force --no-floppy --boot-directory=/mnt/usb/boot /dev/sdc
+
+You should now find a `/boot/grub` directory in that partition with a file
+`grub.cfg`. That's what we will be fiddling with.
+
+If that file doesn't exist, create one with these contents:
+
+```
+set timeout=10
+set default=0
+
+insmod ext2
+```
+
+
+Adding Systems
+==============
+
+As stated above, I prefer to have separate partitions for each system I want to
+boot. Since you can only have 4 primary partitions on a drive, we need to embed
+our additional systems into an extended partition. So, I created an extended
+partition in the remaining 16 GiB of my 32 GiB flash drive. That's where new
+tools will go.
+
+But besides the files, we also need to instruct our newly installed `grub` where
+to look for files and how to boot them.
+
+You can find many boot settings on [this blog post](http://pongup.blogspot.de/2010/10/multipleboot-usb.html)
+already. However, these are for `syslinux`. I'll post some options for `grub`
+here.
+
+Also, the needed files and configuration lines to boot the systems are very
+similar to those of iPXE for network booting. So you can check [this post]({% post_url 2016-03-05-ipxe-scripts-for-tools %})
+and use them.
+
+
+System Rescue CD
+----------------
+
+If you mount the ISO image and have a look at the contents, you'll see that the
+SysRescCD is about 410 MiB in size (all kernels + `sysrcd.dat` and a few
+others). To be safe for future (probably slightly larger) versions, I went with
+a 450 MiB (470 MB[^2]) partition inside the newly created extended partition and
+also formatted it as ext4. For me, the partition is called `sdc7`.
+
+![]({{ site.url }}/assets/multi-usb-partitions.png){: width="800px"}
+
+Copy the following directories and files from the SysRescCD to that new
+partition:
+
+* `isolinux/` (the whole directory incl. all files and subdirectories)
+* `sysrcd.dat`
+* `sysrcd.md5`
+* `version`
+
+Now to find out the boot parameters, we have to look at the boot configuration
+for the SysRescCD itself. This is contained in the `isolinux/isolinux.cfg` file.
+
+Search for these lines:
+
+```
+LABEL rescue32_2
+MENU LABEL 2. SystemRescueCd with all files cached to memory
+LINUX rescue32
+INITRD initram.igz
+APPEND docache
+TEXT HELP
+Boot standard 32bit kernel and run system from memory.
+It requires 512 MB of memory to work and takes some time during the
+boot process, but the cdrom can be removed and system will be faster.
+ENDTEXT
+```
+
+This contains all information we need to create our `grub.cfg` lines. The kernel
+to boot is `rescue32` with parameter `docache`. And the initfs is `initram.igz`.
+
+Also, I have the files in a directory `isolinux` on partition `sdc7`, or #7 on
+the flash drive. Translated to grub-speak, this is `(hd0,msdos7)` (`hd0` means
+the drive we are booting from.)
+
+So the lines we need to add to the `grub.cfg` are:
+
+```
+menuentry "SystemRescueCd 4.9.0 (32bit)" {
+ linux (hd0,msdos7)/isolinux/rescue32 docache
+ initrd (hd0,msdos7)/isolinux/initram.igz
+}
+```
+
+In the same way, the 64bit version with German keyboard layout would look like
+this:
+
+```
+menuentry "SystemRescueCd 4.9.0 (64bit)" {
+ linux (hd0,msdos7)/isolinux/rescue64 setkmap=de docache
+ initrd (hd0,msdos7)/isolinux/initram.igz
+}
+```
+
+You can read about more parameters for the kernel on the [official SysRescCD
+documentation](https://www.system-rescue-cd.org/Sysresccd-manual-en_Booting_the_CD-ROM).
+
+That's it. You should now be able to boot SysRescCD from your flash drive.
+
+
+
+[^1]: If you're afraid about wear&tear with ext4, you could also use ext2 but
+      should `fsck` the partitions every once in a while.
+
+[^2]: "MiB" is MebiByte where 1 MiB = 1024 KiB = 1024 Bytes. Whereas "MB" is
+      MegaByte where 1 MB = 1000 kB = 1000 Bytes. And although most system tools
+      show "MB", some mean "MiB" and others actually mean "MB". And some mix it
+      up completely.