Question: how to transfer files from external hard drive to android tablet?
Answer: Connect the external HDD to the tablet via the Y cable and the OTG cable. ... 
In the file manager app, navigate to the temporal folder (“Day 1”), select all files, and click “copy”.
Go to the HDD folder (again, the folder is normally under /sdcard/usbStorage/sda1, if you use Nexus 7).