The version of Twonkymedia that ships with the current Mycloud firmware is old and has some bugs.
Each time I restart my MyCloudMirror, it hangs whilst creating thumbnails on a MP2 VOB file, it’s always the same file, and always hangs. I have left it weeks pegged at 100% CPU. To sort it out, I have to kill the thumbnailing task and then Twonkymedia moves onto the next file, and eventually finishes it’s startup scan, and the CPU drops to it’s nominal 10%
http://community.wd.com/t5/My-Cloud-Mirror/CPU-pegged-at-100-permemently/m-p/881638#M2126
I am hoping that WD release a firmware update with the latest Twonkymedia 8.1x which fixes this issue (according to the Twonkymedia changelogs).
Time for another round of mycloud firmware beta testing 
