1

flash drives work with OTG adapters but 2.5" HDDs don't.

I thought it's because of the larger power draw for HDDs but then I connected a HDD to a computer through a USB voltage tester and it showed 0.5A power draw. That's almost nothing (the phone charges with 2A). And I tried with a SSD and it showed 0.2A power draw.

When I connected the HDD to the phone via OTG adapter nothing shows up on the phone, but the voltage tester still shows 0.3-0.4A power draw. With the SSD it switches on and off (voltage keeps switching between 0 and 5V)

So what's up with HDDs and phones ?

Improve this question
4
  • 1
    There is a huge difference between charges with 2A and the power requirements for connected devices. This is most likely a power issue.
    – LPChip
    Commented Dec 14, 2017 at 17:39
  • Then how about this: I connected a RTL SDR device via OTG and it works. This device draws above 0.5A !
    – wat
    Commented Dec 14, 2017 at 17:41
  • 1
    My guess is that it's a driver issue. I think SSDs have more on-board processing, and smartphones are built around solid-state technology. While it's common for phones to directly connect to an SD card, directly connecting to a HDD is likely not a use case that designers considered important.
    – Acccumulation
    Commented Dec 14, 2017 at 18:00
  • What phone, does it have a syslog or dmesg to check? What's it's usb port's max power output? Maybe it can see the device, but isn't mounting it for some reason. And how are you measuring it's current draw, inline multimeter?
    – Xen2050
    Commented Dec 14, 2017 at 20:51

1 Answer 1

Reset to default
2

A small battery-powered USB host is called "low-power host", and is obligated to supply only 100 mA per port. See Section 7.2.1 of USB 2.0 specifications. Anything above this could be just a matter of port high-side switch margins and luck.

A USB device can be honest and report its real maximum power consumption (max-power), and if the reported consumption is above 100 mA, the host will likely refuse the connection following its designed power policy. A device that draws more than 500 mA is likely lying to the host, so it gets connected, but sucks all phone power. The SDD is likely marginal, and during peak activity (reading FAT tables) might exceed the limit, and errors prevent the phone to mount it.

In short, small battery-powered gadgets are not designed to drive full-blown USB devices, they are good only to drive light peripherals (is mice/keyboards/pen drives), or self-powered hubs (the ones who don't take power from host). Try to connect your drives to a self-powered hub, and likely you will see a difference.

Improve this answer

Not the answer you're looking for? Browse other questions tagged .