system installation - Ubuntu unattended Server install not recognising seed files when installing from USB

I am trying an unattended install of ubuntu-server-18.04.1 from within windows. Below are the steps I have tried. I am trying all these from within VirtualBox. I have tried with VMWare as well. Both have the same end result.

1. Create an unattended iso from a linux machine using the steps provided by https://github.com/netson/ubuntu-unattended


2. Copy the iso to windows.


3. Shrinked the volume of windows partition from 60 GB and created an unallocated space of 10 GB.


4. Created a new FAT32 partition.


5. Used Universal USB installer to use the ubuntu server unattended iso and write the necessary content to the newly FAT32 partitioned drive.


6. After reboot, the Universal USB installer loads and auto loads ubuntu unattended installer.

However, it does not use the seed file. I have to manually select each option and answer questions in Ubuntu installation.
The same setup works when mounting the iso as DVD in VirtualBox.

The boot params are listed below for reference.

label autoinstall
  menu label ^Autoinstall NETSON Ubuntu Server
  kernel /install/netboot/ubuntu-installer/amd64/linux
  append file=/cdrom/preseed/ubuntu-server.seed cdrom-detect/try-usb=true noprompt initrd=/install/netboot/ubuntu-installer/amd64/initrd.gz auto=true priority=high preseed/file=/cdrom/preseed/netson.seed preseed/file/checksum=36325a6ee7170e69668ebfcc611091b9  /home/andy/iso_new/preseed/netson.seed --

What could be the possible reasons for it?