Multiboot USB

Booting from multiple ISOs on one USB

Setup

Device Discovery

Run fdisk -l:

...
Disk /dev/sda: 30.8 GB, 30752000000 bytes, 60062500 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: gpt


#         Start          End    Size  Type            Name
1         2048       411647    200M  EFI System      EFI System
2       411648     60062466   28.5G  Linux filesyste Linux filesystem

or lsblk -f (we'll be using lsblk later so maybe time to get used to it):

# lsblk -f
NAME                   FSTYPE      LABEL UUID                                   MOUNTPOINT
sda
├─sda1                 vfat              F3AA-CA9F                              /tmp/usb-sda/boot/efi
└─sda2                 xfs               786ac5ec-1d5b-4e8e-871e-cd6c0afd7fe4   /tmp/usb-sda
...

Here we have figured out that the USB stick is sda and can see the result of a previous run of our partitioning efforts.

Note

For anyone wondering how the USB stick is sda, aka. the first disk the answer is that this machine has booted off an NVMe disk which CentOS is picking up as the nvme0n1 device leaving sd<x> available for use for removable media.

Device Reset

We can (perhaps should) clear things up so we have a clean shot:

dd if=/dev/zero of=/dev/sda bs=$((1024*1024)) count=1000

Here, we're slapping NULs over the first GiB which should trample over the MBR and primary GPT blocks. This won't destroy the secondary GPT blocks as they are at the end of the disk. However, we want this to finish today (even if it is USB 3.0).

Warning

If you have a large amount of RAM the command will finish faster than you think as the data is sat in cache waiting to be synced to disk. Wait for it all to finish flushing out:

# time sync

real    0m17.754s
user    0m0.000s
sys     0m0.019s

Check we've cleaned things up:

# lsblk -f /dev/sda
NAME                   FSTYPE      LABEL UUID                                   MOUNTPOINT
sda
├─sda1                 vfat              F3AA-CA9F                              /tmp/usb-sda/boot/efi
└─sda2                 xfs               786ac5ec-1d5b-4e8e-871e-cd6c0afd7fe4   /tmp/usb-sda

Eh? It's all still there! Sadly, the kernel hangs on to the information. We might need to reboot...

# lsblk -f /dev/sda
NAME                   FSTYPE      LABEL UUID                                   MOUNTPOINT
sda

Phew!

Additional Packages

We'll be using gdisk (the GPT equivalent to fdisk) and we'll need some GRUB2 EFI modules for x86_64:

yum -y install gdisk grub2-efi-modules

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