An alternative is to just add a configfile to 40_custom like this:

menuentry 'Live ISOs on SSD' {

configfile (hd0,3)/ISO/livecdimage.cfg

} 



menuentry 'Live ISOs on HDD (boot on SSD)' {

configfile (hd1,3)/ISO/livecdimage.cfg

} 

I do this as when I update an ISO, I invariably forget to run sudo update-grub. The above entries never have to change and I can just edit livecdimage.cfg which has the same format as any 40_custom but without header lines, and is in my /ISO folder in my ISO partition.

I do add toram as another boot parameter and on the system with Nvidia add nomodeset boot parameter. But still often have to unmount the /isodevice.

Unable to umount isodevice unmount ISO
https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1155216

sudo umount -l -r -f /isodevice

And this is part of my livecdimage.cfg file

# livecdimage.cfg

# Add this to 40_custom to load this file:

# menuentry 'Live ISOs' {

# configfile (hd1,3)/iso/livecdimage.cfg

#} 

# Add iso names to livecdimage.cfg

#for i in `ls *.iso`;do echo "# "$i>>livecdimage.cfg; done;



menuentry "Ubuntu 16.04.4 xenial amd64" {

    set isofile="/ISO/ubuntu-16.04.4-desktop-amd64.iso"

    loopback loop (hd0,3)$isofile 

    linux (loop)/casper/vmlinuz.efi boot=casper iso-scan/filename=$isofile toram

    initrd (loop)/casper/initrd.lz

}



menuentry "Ubuntu 18.04 Bionic amd64" {

    set isofile="/ISO/bionic-desktop-amd64.iso"

    loopback loop (hd0,3)$isofile 

    linux (loop)/casper/vmlinuz.efi boot=casper iso-scan/filename=$isofile toram

    initrd (loop)/casper/initrd.lz

}



# spacer line

menuentry " " {

set root= 

}



menuentry "Reboot" {

    reboot

}



menuentry "Halt" {

    halt

}

Three easy steps

to add ISO to the grub menu and boot from it.

First, install grml-rescueboot

Open a terminal and enter:

sudo apt install grml-rescueboot

Second, move the iso files to /boot/grml/

In the same terminal enter:

sudo mv ~/Downloads/<filename.iso> /boot/grml/

Third, update grub

In the same terminal enter:

sudo update-grub

Repeat second and third steps as and when necessary

Don't forget to delete obsolete ISO files from /boot/grml.

Reference: Ubuntu help on Grub2 ISO boot

Hope this helps

mkusb MULTIBOOT HACK

Mkusb makes a great base for custom boot drive projects because of it's abilities with both BIOS and UEFI.

This hack boots multiple ISO files including the Windows installer, and has a grub2 menu.

Grub2 menuentries for most OS are available using Google Search and are beyond the scope of this answer.

Use mkusb defaults when creating the Persistent USB drive.

Use mkusb persistence defaults if in doubt, partition size can be adjusted later but takes time.

The DUS Console after install.

GParted before partition modification.

Gparted after modification - Overwrite sdb4, the ISO9660 OS partition and sdb5, the ext2 casper-rw partition, with a FAT32 partition for persistence files.

Persistence partition - Create an uniquely name folder for each OS, (that requires persistence)

Persistence folder - Add a casper-rw file and optional home-rw file to each persistence folder. A home-rw file can be made by renaming a casper-rw file. A home-rw file is like a seperate home partition on a Full install, it can be reused after version upgrades.

Create a folder for the ISO files on the NTFS usbdata partition.

Add some ISO's to the ISO folder.

grub.cfg location

Edit grub.cfg to loopmount the ISO files. Include: persistent persistent-path=/<persistent-folder-name>/ if you want persistence.

sudo parted -ls /dev/sdb

sudo lsblk -f /dev/sdb

If a Windows installer is required it is possibly easiest to start with the mkusb "extracting Windows installer" function, (I had to extract the Windows ISO to TAR manually),

After installation create a folder for ISO's and if required, folders for persistence, (similar to above procedure).

Edit /boot/grub/grub.cfg to loopmount any ISO's and specify any persistence folders.

(have not found a way to loopmount a Windows ISO file).

If the above is used as a USB stick it can be used to boot ISO's stored on a Windows only computer. Grub is not required on the internal drive.

MultiBootUSB - QEMU

If you want to boot ISO files with the least hassle and don't need persistence, MultiBootUSB includes a QEMU option:

• Start MultiBootUSB and select the Boot ISO/USB tab.




• Drag and drop the ISO on the Select image space.




• Select RAM size and hit the Boot ISO button.

You don't need to edit grub.cfg or even log out.

SIMPLE mkusb ISO multibooter

If you don't need multi-persistence, it is easy to multiboot operating system ISO's on a mkusb flash drive.

Use mkusb to make a Persistent USB drive using a default OS of your choice, https://help.ubuntu.com/community/mkusb

Make a folder in the usbdata partition sdx1, named ISOs.

Add the ISO's to be booted to this folder.

Add menuentries to /sdx3/boot/grub/grub.cfg to loopmount the ISO's.

Samples:

menuentry "Ubuntu-18.04 64-bit ISO" {

    set root=(hd0,1)

    set isofile="/ISOs/ubuntu-18.04-desktop-amd64.iso"

        loopback loop $isofile

        linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$isofile splash --

        initrd (loop)/casper/initrd.lz

}



menuentry "GParted 64-bit ISO" {

    set root=(hd0,1)

    set isofile="/ISOs/gparted-live-0.31.0-1-amd64.iso"

    loopback loop $isofile

    linux (loop)/live/vmlinuz boot=live union=overlay username=user config components noswap noeject toram=filesystem.squashfs ip='' nosplash findiso=$isofile splash --

    initrd (loop)/live/initrd.img

}



menuentry "Clonezilla 64-bit ISO" {

    set root=(hd0,1)

    set isofile="/ISOs/clonezilla-live-2.5.5-38-amd64.iso"

    loopback loop $isofile

    linux (loop)/live/vmlinuz boot=live live-config nolocales edd=on nomodeset ocs_live_run="ocs-live-general" ocs_live_extra_param=""   ocs_live_keymap="" ocs_live_batch="no" ocs_lang="" vga=788   ip=frommedia nosplash toram=filesystem.squashfs findiso=$isofile splash --

    initrd (loop)/live/initrd.img

}