Try the following:

• Switch off and then turn on the device.
• Remove the SD card and insert again. 

The content in SD card can be found under “ES files explorer -> external_sd card folder”.

If the SD card still cannot be found under ES file explorer, follow the steps below to verify whether it is the SD card, or the card slot in the tablet, has problem:

• Insert your SD card to a PC to see whether the card is found. If it is not found, it is probably that the SD card is defective.
• Try another SD card with the tablet. If the SD card is also not found, it is probably that the card slot has problem. Contact Philips Consumer Care (Philips Support Page)  for assistance.