Can't play videos. I upload my Mp4 with videos,each one on a different format ,to make sure it

Question

I upload my Mp4 with videos,each one on a different format ,to make sure it support these format as it said on the instruction,but when i try to open a video on my Mp4,the error File format error appears.I opened the removable disk and i can see all the videos loaded there,in the Mp4 8Gb!!(from Hong Kong,same as an Mp3 8GB)I even try to sync. the videos with wind.media player 11 and the Mp4-the synchronization is successful,the name of the videos is listed on the mp4 explorer but still,same error?What am i doing wrong/Please help me..many thanks

won't play on unit