Reading the Parted Magic Frugal install instruction did get me some valuable information :
https://partedmagic.com/frugal-install/
Simply put, the boot process for Parted Magic will seek the Squashfs in removable devices, which do not exist in a “sanboot” :
root=/dev/sdXX
Starting with PartedMagic 6.2 the search for the pmagic-.sqfs file will begin with the removable devices, unless a device/partition (root=/dev/sdXX), label (label=XXXXXXXX) or partial uuid (uuid=XXXXXX) is stated. Without such a kernel boot parameter a 10 seconds delay will occur.*
Furthermore, the boot must be set to “boot=/boot” :
directory=/boot
Previous versions of this document said to copy the “pmagic” folder to ”/”. This carried the risk of a subsequent run of PartedMagic from a CD or a USB stick to find the pmagic-.sqfs file on the disk instead of the on the media – something you would not want in the majority of cases.
Starting with PartedMagic 6.2 the risk is inverted: a subsequent run of PartedMagic from the disk might find the pmagic-.sqfs file on the media.
The solution in both cases is to “hide” the “pmagic” folder on the disk in e.g. the ”/boot” folder.