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I have a Raspberry Pi 3 to which I need to plug an external HD. Such HD is a WD My Passport, which is powered directly from USB and it works perfectly with my laptop. At the first connection attempt, the HD led didn't even stay on. Conscious of the power limitations of the Raspberry Pi 3, I maximized the USB current output by adding the line max_usb_current=1 to /boot/config.txt Now the HD's led stays on and is correctly recognized by the lsusb command as follows:

Bus 001 Device 004: ID 1058:25e2 Western Digital Technologies, Inc.
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet Adapter
Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp. SMC9514 Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

However, I cannot find the disk anywhere in the system. Indeed if I look at dmesg this is what I observe:

[  260.551388] usb 1-1.3: new high-speed USB device number 4 using dwc_otg
[  261.483575] usb 1-1.3: New USB device found, idVendor=1058, idProduct=25e2
[  261.483586] usb 1-1.3: New USB device strings: Mfr=2, Product=3, SerialNumber=1
[  261.483595] usb 1-1.3: Product: My Passport 25E2
[  261.483602] usb 1-1.3: Manufacturer: Western Digital
[  261.483610] usb 1-1.3: SerialNumber: 575832314443364632304C36
[  261.484621] usb-storage 1-1.3:1.0: USB Mass Storage device detected
[  261.490655] scsi host0: usb-storage 1-1.3:1.0
[  262.562475] scsi 0:0:0:0: Direct-Access     WD       My Passport 25E2 4004 PQ: 0 ANSI: 6
[  262.565247] scsi 0:0:0:1: Enclosure         WD       SES Device       4004 PQ: 0 ANSI: 6
[  262.565731] sd 0:0:0:0: [sda] Unit Not Ready
[  262.565745] sd 0:0:0:0: [sda] Sense Key : 0x4 [current]
[  262.565757] sd 0:0:0:0: [sda] ASC=0x44 <<vendor>>ASCQ=0x81
[  262.584641] sd 0:0:0:0: Attached scsi generic sg0 type 0
[  262.652202] scsi 0:0:0:1: Attached scsi generic sg1 type 13
[  442.591949] sd 0:0:0:0: timing out command, waited 180s

If I touch the HD, I can feel it doesn't spin. Also, it occasionally beeps every random amount of minutes. Could this really be a power issue? As far as I know, enabling max_usb_current=1 should provide currents up to the USB3.0 standard. The HD is the only thing connected to the Raspberry Pi 3 and the power supply provides 2.1A.

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3 Answers 3

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Nothing is going to make the Pi USB2 "provide currents up to the USB3.0 standard"
The Pi is not even compliant to the USB2 standard, and supplies current without negotiation.

Maximum total USB peripheral current for Pi(2/B+) is 600/1200mA. The limit for Pi3 is 1200mA. This current can be drawn from a single port, or shared.

See Raspberry Pi Power Limitations for detail of the Pi power.

NOTE the Pi2/3 SHOULD be able to power HDD, but my experience indicates this is not so.

I have tested, and it is indeed possible to draw the full current, but NOT at the USB specified voltage. It is a combination of PSU which don't actually supply the correct voltage, thin USB power cable and internal voltage drops.

I have never been able to reliably power a USB HDD with an official PSU. Measurements of the voltage supplied confirms this. This is not so much a failure of the Pi itself, the majority of cables, including those wired to the official PSU use quite thin cable. I always use a powered hub.

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A common way to avoid USB power limitations on the Raspberry Pi is to used a powered USB hub, home that has its own power supply from the wall.

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Solved by purchasing a better USB cable to connect the Raspberry Pi 3 to the power adapter. Apparently the previous one was giving a big voltage drop.

If you step on the same problem, ensure you have a proper power supply. The official Raspberry 2.5A adapter works perfectly with me so far. Also, ensure that the red led on your Raspberry Pi is always ON. In my case it wasn't before, but I found out it signals that the input voltage is correct. So if it's not, your Raspberry Pi might not be fed properly.

If you don't like the official power, try using a better quality USB cable for feeding the pi: before buying the official adapter I switched from a common 28AWG (standard size), to a 20AWG (much thicker) and the HD was working. However I had occasional freezes, which disappeared after switching to the official power supply.

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