When you plug an external hard drive on your device via a USB host, it should be displayed in the Files application. If it is not the case: 

• Make sure your device is compatible with the function "host" in the technical specifications available on our website
• If the USB host is via micro-USB, make sure to use a compatible USB OTG cable. 
• Also make sure, in case your device has two USB ports (one for your computer and one with the funciton "host") to use the right port. 
• Most of external hard drive require a power supply higher than the one provided by the USB port on your device. It delivers maximum 500 mA. Read the specifications of your hard drive and plug it to an exteranal power supply if needed. 
• Also make sure that your hard drive uses the right files system. By default, your device recognizes only the storage solutions formatted to FAT32.