Ok, you give me tow tips to try this evenning.
Change the max transfert value.
I will try "0x0001FE00" that seem to be a "safe" value for unknown disks.
http://www.wonkity.com/~wblock/a1200/a1200.html
I suspect this can be a very good advice because it look logic in front of the symptoms.
I will try to format without dircache too.
Other tips are welcome.
Thks.