About a week ago I started using a magnetic case with my Nexus 7 (Slim Fit Leather Case by SupCase-http://www.amazon.com/Google-Nexus-Tablet-Leather-Supcase/dp/B008HDLI9Y/) and shortly thereafter, the tablet would randomly go into sleep mode when I had the front cover folded behind with the magnets now on the backside of the magnetic edge of the tablet. 

Although I can feel no magnetic attraction between the tablet and the cover magnets through the two layers of leather in this position, I have never experienced an unintentional sleep mode activation with the cover open to the left, nor did I in the week and a half I used the tablet prior to getting the case.

Although it's all circumstantial evidence at this point, I'm wondering if the magnetic latch on the tablet is so sensitive that merely having magnets in its vicinity is enough to trigger activation of the sleep mode.

If anyone else has had a similar experience, or perhaps some other evidence, I would like to hear about it.

Do you have SuperSU installed? I would open SuperSU and look under the Apps tab to see if StickMount is registered, and if so, click on its entry to see if has been granted (superuser) access rights.  If not, you should be able to grant access at that point.

Also, on my tablet, the USB drive was mounted on /storage/sdcard0/usbStorage/sda1 rather than on /mnt/usbdrive/sda1, where I expected to find it.