I also successfully rooted my tmobile M919 on 4.4.2 with towelroot. I then installed SU from chainfires site and next installed CWM from ROM manager. My knox counter is tripped, but oh well.
The thing I have noticed is that, since root, some applications that want to access the SD card are having trouble. For instance, Flipboard opens and runs ok, but there are no pictures. Kindle reading app cannot downnload my book to the device. It will sync them, but hangs when asked to download them. ESPN app would not download podcasts, UNTIL I switched the storage location from external to internal storage. (I ttried to change the Cache from external to internal in flipboard, but the application force closes everytime i try to make that switch - another indication that certain apps are having trouble with extrnal storage.
I have tried another SD card, but still no go.
Any ideas? Thanks
(also, if I were to decide to unroot, would the simplest way be to do it through SU ?
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