Ok i think i have identified a bug. Its a common one but i have been able to replicate behavior and have a temporary solution.
The problem is when u connect Vox to pc and turn on usb for sdcard access. Well sometimes when u eject internal and external sdcard from pc using eject usb device, then click on turn off usb in Vox. Then Disconnect usb cable. There is an error and one or both sdcards become inaccessible. This results in the Vox being unable to read either drive.
Going into settings and unmounting then remounting sdcards will fix this. I find it useful to reboot afterwards just to be sure. Their is no error message that i can see that indicates a problem. The way i tell is the kobo ebook widget stops showing ebook covers. I have a rooted Vox with the beta update.