Just a couple of suggestions:
Turn off the DX50 and remove your SD card
Turn it back on and do a Factory Reset from the Settings/Advanced menu
After the factory reset has been completed, re-insert the SD card and allow the player to complete the first scan. It might take 20 minutes or so. Go have a cup of coffee and don't interrupt the scan, as it will then start over next time.
With the latest firmware, after the initial scan, the Media Scanning screen should only be in view for about 4 or 5 seconds after booting.
Unless using OTG, it is best to use All Music, Album, Artist, or Genre rather than the Directory, as Directory has limited functions and will only scan whatever file you select for playing. There is no way to enable a full OTG media scan nor is there any way to save OTG files to Playlists or even randomly play all files on the OTG drive.
I format my cards to FAT32 as my computer cannot read exFAT. All cards 64gb and larger come formatted in exFAT. Keep in mind that .wav files will not display any album art.
Hope this helps.