Trouble with using Seagate 2TB backup lus desktop on HDTV

Hi, im having a rather irritating problem, i have a Philips HDTV that i can use usb hard drives to play videofiles, 

my Iomega Ego 500Gb usb diskdrive uses no external power and runs all video files up to 20gb just fine, mkv, avi, mpeg4 and more.

But i recently bought a Seagate 2TB Backup plus desktop HDD with external power and im having issues with the bigger files, it plays files in 720p around a 2-4 gb just fine, but my other encoded files from 5-15 gb just wont play and one i got sound but no picture and the iomega plays it just fine. any tips on what the problem is and can be solved?

You might be able to get an answer to that in a Philips or Seagate forum.