So is it just a question of cover the High Density hole insert in the Amiga and click format? Or I still need CrossDos or some other tool? My aim is to have disks which I can boot from. From what I found out on the net, using CrossDos to format a disk means you'll need CrossDos to read the content of that disk and therefore impossible to use such disks unless in Workbench.
That's the problem, as long as your Amiga drive is in 100% great shape. The high density disks have more tracks, that are much smaller than the double density disks - so alignment, etc. is more important than ever.
But as long as you cover that HD hole, they should work on your Amiga.
PS - do not use transparent tape! The drive may still "see" the hole. Use electrical tape.