I have a 250GB Western Digital Passport, 1TB Samsung M3 and a 500GB DIY portable HDD I made with an old laptop drive and USB 3.0 enclosure from eBay... and none of them require an external power supply as they're all USB powered.
Also, when I had a Note 2 I could connect these drives to it... so whilst I can't say for sure they will work with the N5, I don't think it should be ruled out.
I'm at work right now and have no idea where my USB OTG cable is, but if I can find it when I get home I'll test them out.
Main Device: Google Nexus 5
Other Devices: Samsung Galaxy Note 2, Samsung Galaxy Tab 2 10.1, Samsung Galaxy S2, Motorola Moto G
Retired: HTC Hero, HTC Wildfire S, HTC Desire HD, Samsung Galaxy Note 1
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