If the content on the flash card in in a subfolder, then the BrightSign won't "see" the content at startup. 

1. Verify content is in the root of the flash card
2. Verify the card is inserted properly into the sd card slot until you hear a click
3. If this is a new card, please reformat as fat32
4. Reformat your SD card in FAT32 format.
5. Try a different brand and/or size card.
6. If using an HD2000, please verify it's an SD card, not SDHC.
7. If using an HD600, SD cards are not supported.