This page is one way of creating a bootable USB drive for Sophos UTM 9.5. This was done from a Linux terminal using a Sophos UTM 9.5, "asg-9.506-2.1.iso", file. This way worked for me and is based on information gathered from various web pages relating to creating bootable USB flash drives and gettting the Sophos UTM 9.5 to install from a flash drive. These instructions were performed using Ubuntu 16.04.3 LTS, and should work from most debian based distros with little or no changes.

Create Bootable USB Flash Drive

• Insert a USB flash drive (a 2GB or larger flash drive will work).
  
  
• Note: These steps will completely overwrite the contents of the USB flash drive!
• Note: These steps will destroy any existing partitions on the USB flash drive!
• Note: Make sure you're using the correct device path! Using the wrong device path may destroy the contents of your hard drive!
• Note: These steps worked for me, but your results may be different. Proceed at your own risk!
  
• Note: In the examples below, /dev/sdZ is used as the device path. Replace /dev/sdZ with your device path.
  
  
• Note: You can list the drives on your system to find the flash drive device path using "lsblk" from a command prompt to list the drives on your system.
  
  
• From a terminal, unmount the USB flash drive if it is mounted.
  • sudo umount /dev/sdZ
• Install the syslinux utilities
  • sudo apt install syslinux-utils
• Create a filesystem on the usb flash drive
  • sudo mkfs -t vfat -I /dev/sde
• Modify the iso file for booting as a hard disk specifying partition 4, hidden fat32 partition type
  • isohybrid asg-9.506-2.1.iso --entry 4 --type 0x1c
• Copy the iso file to the device path (Note: Use /dev/sdZ and not /dev/sdZ4))
  • sudo dd if='asg-9.506-2.1.iso' of=/dev/sdZ bs=8M
• Synchronize (flush) any cached writes to the USB flash drive
  • sync

Installation: "install.tar wasn't found" Error

Note: The installation from the USB flash drive will fail with an error stating that the install.tar file wasn't found. Follow these steps to correct the problem.

• At the installation start screen, press Enter
• At the instroduction screen, do not select Start. Instead, press Alt+F2. Then at the root prompt, enter the following command.
  • mount -t iso9660 /dev/sdZ4 /install
  Note: Remember to substitute your device path for /dev/sdZ4. In this case, we need to specify the partition number.


• Then leave the bash prompt, press Alt+F1. Then select Start on the introduction screen and proceed with the install.