First I would recommend removing and reinserting the usb drive to see if that fixes the issue.
This could be due to an issue that existed in .deb installations prior to v0.10. It was due to the
system service, the script that runs Kolibri, being owned by a
kolibri user account, rather than the desktop user account. This led to external drive access issues.
The solution involves changing the owner of the
system service. The steps for this can be found in the Kolibri Debian/Ubuntu installation docs.