This is one of several ways to do this:
Create a new dos partition table.
Create the first partition on the usb 3GB formated fat32 and set the boot flag on the partition.
Extract the contents of the debian xfce live iso to the partition.
Mount the first partition
As root in a terminal run
Code:
grub-install --target=i386-pc --boot-directory=media/arthur/multiDT3/boot --recheck /dev/sdb
I used your setup mount point and sdb for the usb as an example, change accordingly if different.
If you get a no i386-pc error with the grub-install, run
Code:
apt install grub-pc
as root then rerun the grub-install command, assuming your using a debian based distro to do this.
This will get the debian live iso to boot in both efi and legacy mode.
Create a second partition to store iso's.
Create configfile in the root of second partition with the menu entries to boot your iso's. You can name the grub.cfg to something else to better identify it, maybe isomenu.cfg
add something like this to the end of /boot/grub/grub.cfg on the first partition
Code:
menuentry 'iso configfile'{
search --no-floppy --fs-uuid --set=root <uuid of second partition>
configfile /isomenu.cfg
}
