Why my system boots no more when I add a USB disk ?

[Pierre Couderc]

It is a "simple" system with legacy boot and btrfs disks (described here 
https://wiki.couderc.eu/doku.php?id=public:a_perfect_btrfs_server).

"simple" but I had to force grub reinstall for it to boot first time.
Now it boots very fine.

But if I connect a USB hard disk  (formatted in bftrs with no GPT, 
accessed by "mount -t btrfs /def/sda"  ), it boots no more.

I suspect it could be a question of drive numbering as fdisk -l detects 
the USB disk as /dev/sda.

Thanks fopr any help

PC