The Movidius USB stick disappears from the list after one completed execution of any program in the NCAPPZOO folder (or any program accessing the stick to perform operations on it), this happens on a MacOS High Sierra laptop, with Ubuntu running on a VirtualBox.
On replugging the stick back to the machine, it becomes visible again but repeats the above behaviour.
After one execution of an example, your NCS device will need to reboot. If you don't have USB filters set up, the NCS device will connect to your host machine instead of your VM. To set up USB filters, follow the directions at the bottom of this page:
Please let me know if this information is helpful.
Thanks for reaching out. In addition to @Sahira_at_Intel suggestion, I've worked a very similar issue and recommended a few things in this post https://ncsforum.movidius.com/discussion/1443/error-could-not-open-ncs-device#latest.
I'd strongly suggest to take a look and let us know if it resolved your problem.