I have encountered this issue when flash my Nexus7 Flo on Ubuntu 12.04LTS. As a result, it kinda happens to lots of people. The major problem that causes this was the Linux permissions. As a user, you are not running as root user, therefore cannot access the USB port that connects to your devices as an admin.
To solve this:
1, add yourself in root group, I don't suggest you do that, that's going to open security holes.
1, add yourself in root group, I don't suggest you do that, that's going to open security holes.
2, better way, type "sudo su" in your command line to temporarily login as admin/root. Now you may run fastboot soomthly!
As always, you are welcome.
