Thank you, it works better that way 
mkdir /volume1/usb
nano /etc/fstab
/dev/sdb1 /volume1/usb vfat defaults 0 0
mount /volume1/usb
But after a reboot, the line does not remain written in fstab and not anything in the external devices under DSM once mounted.
Is would it be possible to fix this problem?
DSM really great, it’s really sad that he does not work on Debian, completely lost with Unix.
Edit :
Try /etc.default/fstab
I had not understood, sorry for my ignorance.
But it does not work, I added :
nano /etc.defaults/fstab
/dev/sdb1 /volume1/usb vfat defaults 0 0
The volume does not mount automatically.
I just read this topic, drivers problem? => Synology OS on WD my Cloud - new Hope - #30 by seedg4
