Nexus 7 2nd gen not picking up flash drive

[AC Question]

Help needed. I have a rooted nexus 7 2nd gen on 6.0.1 but it doesn't pick up the usb flash drive. Picks up my mains powered hdd but not the flash drive. The same flash drive and otg cable are recognised on my 1 st gen nexus and on my nexus 10. It's like the 2 me gen doesn't want to power the usb. I've run root check and it's rooted and I've run otg trouble shooter and that says otg is OK but can't detect flash drive. Any help appreciated. Thanks