Does Not recognize hard drive

2 Days ago I connected my external hardrive (2TB Seagate) to my X96 android box. I populated my library with music and movies. The next day I decided to take my hard drive to my computer and add some more movies and music. I connected back to the android box but it suddenly cannot recognize the hard drive anymore and therefore will not play anything. Is there something Iam doing wrong? what is the best way to add files to your library when using a external hard drive?. I have another box (mecool proB) with a 1TB portable Toshiba which is doing the same so I assume it has something to do with disconnecting from the box and adding files to it.