You might have some filesystem problem on your 450X2 internal disk.
Just connect it by usb cable to a pc and do a check of the filesystem (windows explorer, right click on the drive letter and use tools tab do check the filesystem of the disk.
Before disconnecting the usb cable of your mede8er use the safe removal tool:
Win7, 8, 8.1 users - Safe Removal is incompatible with the linux NTFS code. Instead use Utility to Eject the Mede8er (or shared external USB drive) - USB Disk Ejector http://quickandeasysoftware.net/software/usb-disk-ejector
Vanishing files and un-mountable partitions can occur as a result of using Safe Removal. Those should get fixed by using slave mode, running chkdsk and removing the player using the USB Disk Ejector tool.