Hi Ian, thank you very much for your answer.
The problem is heaven more complicated :
I installed PHPvirtualbox under Open Media Vault.
When I start the server, as soon as the virtual machine runs, it
crashed all other
services (can't log uder SSH); but the odd thing is that the virtual
machine
runs perfectly ! (I installed Team Viewer on it)
So it seems that my last operation (giving access to the usbkey boot
system) makes
a trouble. Witch is normal cause 2 systems are sharing the boot key.
When I boot my OMV ...
So, thanks to you, I've been able to avoid running the Vbox servervice
at boot,
then, I booted and changed the directory where the VM was,
and now, I can boot, and I'm reconfiguring the USB access inside  that
VM.
Hope it will work.
Thank you very much!
SAM