Ok if the mp4 is saying "Disk Error" after everything you try to access on the player then you need to format it again but it's gonna take some time. You'll need to format it twice with two different formators. I had to download the HDD Low format tool. You can get it here:
HDDGURU: HDD Low Level Format Tool
You have to download this and do a low format with this first. It might take a while usually it seems to take about 30 minutes for me. After that format is done go into the formator it gave you and format it on full format. After that it should work.
Hope this helps.