I figured it out.
I found, that EM Mode is a old classic in the buffalo world.
After several trial and errors, I make a workaround.
I formatted all the hdds in EXT3. (takes a damn while)
I started the updater in debug mode
Title = BUFFALO TS-IX Series Updater Ver.1.66
WaitReboot = 1200
WaitFormat = 1200
WaitDiscover = 300
WaitFileSend = 1200
ProductID = 0x0000200E
ProductID2 = 0x0000200F
Name = TeraStation
VersionCheck = 0
NoFormatting = 0
iSCSI = 1
checked on the right side to ignore the version, rebuild the partition, forcing the update:
It Takes a while, then comes an error: cant find the partition and aborting the flash/update=
Restart the machine, holding the function button for five seconds, while NAND Flash initialize, and then, leave the machine alone.
It takes a looong time with no informations on the display. But be patient.
Coming to an happy end.