Thank you for your replies,
Tried other cloning methods, used a raw image this time and still end up stuck at DMI Pool verification==> missing operating system.
Cable should be ok, had to replace it as original one broke (old plastic got brittle and broke), I don't think the problem comes from the new cable and it was installed correctly as shown in your attachment and the original HDD boots fine with it.
I really have the impression that my cloning went fine but for some reason the CMOS doesn't update with the new HDD.
Found an AT/PS2 adapter on amazon but I have to wait 1 week to get it.
If I reset the CMOS by removing the battery 5 seconds or set the jumper to CMOS reset (If present on that motherboard) would I risk bricking it as it still boots with the original hdd for the moment?
I have no idea to which bios settings it would reset the motherboard and as it's not a common PC I don't want to mess anything up