USB On-The-Go, often abbreviated to USB OTG or just OTG, is a specification first used in late 2001, that allows USB devices such as digital audio players or mobile phones to act as a host, allowing other USB devices like a USB flash drive, digital camera, mouse, or keyboard to be attached to them.
Asus Zenfone 2 Support USB OTG, but the power is very low (Arround 0.3 A). And to perform USB OTG your Flash drive/External Hard disk must be NTFS format. To perform USB OTG you must have OTG Cable which not included inside Zenfone 2 box.
microUSB Power Test
Read Flash drive via OTG
When open Flash drive with USB OTG, Zenfone 2 can do it without additional power. To open Flash drive go to File Manager and select USBDisk. To eject Flash drive, go to Settings>Storage>Eject USB Disk
Read External Hard disk with OTG
To read External Hard disk with OTG, Zenfone 2 need additional power (charger/power bank) and Y-OTG cable.