Debian – How to create a bootable USB stick with Debian Squeeze

Since you are booting netinst you could skip the ISO entirely and boot the kernel+initrd directly from your USB filesystem. This is a netboot d-i, it will just load the rest of the components from a debian repo.

About the iso search, the process goes something like this:

• grub loads the ISO and reads the kernel and initrd into memory (in your case the initrd was already extracted to the USB drive)
• grub hands control over to the kernel
• the kernel hands control over to the initrd's init script
• the init script hands control over to the debian-installer
• the iso-scan d-i module mounts all available drives looking for the iso-scan/filename provided
• once the ISO is found and loopback-mounted the debian-installer continues with the process

(see also section 6.3.1.4 on the install docs for more details)

When you give a path like /dev/disk/by-uuid/UUID/debian/debian8.iso it doesn't make much sense, since /dev/disk/by-uuid/UUID is a block device and not a mounted filesystem where files can reside.

If you want iso-scan to narrow the search down, besides using iso-scan/filename you can also preseed the values* shared/ask_device=manual and shared/enter_device=/dev/disk/UUID (just add them to your kernel line)

This should be your grub.cfg then:

set imgdevpath="/dev/disk/by-uuid/UUID"

menuentry 'Debian 8.2 Multiarch' {
    set isofile='/boot/iso/debian/debian-8.2.0-amd64-i386-netinst.iso'
    loopback loop $isofile
    linux (loop)/install.amd/vmlinuz iso-scan/filename=$isofile shared/ask_device=manual shared/enter_device=$imgdevpath no-prompt no-eject
    initrd (loop)/install.amd/initrd.gz
}

Note that I haven't really tested this, I'm copy-pasting from a similar setup of mine for an older debian release.

I recommend you start with this and only if it doesn't work try the other initrd. Also make sure it works "manually" before readding the probe for the UUID.

_{* These are the ones I referred to as "hint" before. Guess I remembered hint from ubuntu or some other distro}