Flashing question mark at startup
iMac 24", model A1225 (iMac 9.1), January 2009
500 GB HDD (not original) reformatted GUID, probably SSD
RAM : 2x4 Gb : not original
Wired Ethernet connection in order
Target mode boot on my MBP / High Sierra : HD visible, formatable, readable, writable
Boot via Cmd+R : Flashing question mark
Boot via Cmd+Alt+R : Flashing question mark
Boot via Shift+Cmd+Alt+R : Flashing question mark
Start via D (hardware diag) : Flashing question mark
Booting via Opt+D (online hardware diag) : Flashing question mark
Installing an OS 10.11.6 (dmg saved on the HD of the iMac and opened via MBP) : impossible --> no OS present
Recovery Disk Assistant : impossible --> no OS present
PRAM : zapped
SMC : reinitalized
No corresponding Time Machine available.
I tried to start with the TM of my Mac Mini / High Sierra : the TM is seen but it stops there. The question mark comes back (which seems normal to me : OS not supported)
How can I get this computer back up and running?
Thanks in advance...
Sergio