Firmware 9.0.1

[Stigmater]

you can't update 2 Devialets from the same SD Card. After the first amp has been updated it is deleting some files. If you use this card in the second amp it is updating fast but in fact it is not updating anything. YOu have to copy all files again to the sd card before you update the second amp. Maybe this was the problem ?
It will also take more than a minute on a 250 if it works.

This is correct, during the firmware update the Devialet will delete a file, this is so you can have both the firmware and the configuration on one card.  First the firmware is loaded, the file is deleted so the firmware is not reloaded, then the Devialet can pick up the configuration from the same SD card.

As an alternative for dual mono, you can load the firmware onto one SD card, lock the card to prevent the file deletion, and then use this card to update the firmware on both amplifiers.  This appears to work OK.  Then use separate SD cards for the configuration.

Thanks , another great advice