I am new to Linux and have been reading and trying lots of alternatives before writing here.
I am trying to install Debian10 (through NETINST) from a USB stick on a Laptop that normally runs on Gallium OS. I need to provide non-free firmware in order to use wifi, hence the other partitions.

My process is:

 - wipe my USB Stick with **dd if=/dev/zero of=/dev/sda**
 - create partitions with **fdisk** and assign a bootable flag to the partition that will have the debian install ISO file (**/dev/sda2**)
 - use **isohybrid --partok** on the **debian-10.9.0-amd64-netinst.iso** file
 - use **dd if=debian-10.9.0-amd64-netinst.iso of=/dev/sda2** to create the bootable Debian installer
 - copy the folder with non-free essential firmwares into the other USB partition

When i do the same (without the non-free firmware) on an unpartitioned USB stick, the GRUB installer starts correctly with a graphical interface.

When i do the above (i.e. when trying to boot/install Debian from a partitioned USB) the system displays a GRUB command-line that i cannot use (despite trying browsing contents of the usbstick, i cannot find either vmlinuz nor initrd.img). I am not an expert of GRUB through command-line so maybe I am doing something wrong there, but I cannot get past this phase. 

Ideally I would like to boot into Debian through a graphical intallation.

Could anyone see what may be wrong ?

Thank you!
Jon