USB drive disconnects after 90 or so minutes

Well, there is a year-old unfixed issue with USB3-drives (incl. MyBooks) connected to the backside USB-port: http://community.wd.com/t5/WD-TV-Live-SMP-Discussions/Intention-to-finalize-a-rear-USB-port-issue-drives-USB-3-0/td-p/392589/page/13 Maybe that causes your problem during playback. Other than that, I also have the problem of the device refusing to recognize USB-sticks and self-powered USB-harddrives after a long idle period. The file browser would show the files on the drive, but starting playback would result in a “please eject safely”-error, although I never unplugged the USB-devices. I reported this almost a year ago, and the bug is, like the USB3-bug, unfixed. Take that as you will.