Power external HDD: safe to connect both to RPi and charger?

[szhorvat]

I have an external hard drive that I would like to connect to the Raspberry Pi. This hard drive has a single USB connector and must be powered through it. It came with a cable that has two USB A type ends to be connected to two USB ports of the same computer, for sufficient power.

The Raspbery Pi cannot give sufficient power to this hard drive, even when using both of its ports.

Question: Is it safe to plug the data end of the USB cable into the Raspberry Pi, and the power-only end into a USB wall charger? Can this cause hardware damage to either the Raspberry Pi or the hard drive?

[elatllat]

Hardware damage is unlikely bu if you want to be sure you can block the last contact on the usb cable with some paper and tape, isolating the power from the pi.

[totoharibo]

Hardware damage is unlikely bu if you want to be sure you can block the last contact on the usb cable with some paper and tape, isolating the power from the pi.

IMHO the solution is a powered USB HUB which powers the RPi and the DD.

[FTrevorGowen]

I have an external hard drive that I would like to connect to the Raspberry Pi. This hard drive has a single USB connector and must be powered through it. It came with a cable that has two USB A type ends to be connected to two USB ports of the same computer, for sufficient power.
The Raspbery Pi cannot give sufficient power to this hard drive, even when using both of its ports.
Question: Is it safe to plug the data end of the USB cable into the Raspberry Pi, and the power-only end into a USB wall charger? Can this cause hardware damage to either the Raspberry Pi or the hard drive?

Not really a "safety" issue but rather one of stability, risk of data loss etc. Whilst the use of a very good powered hub**, powered by a 5V 2A PSU (or better) is one solution another possibility is to power both the Pi and HDD from the same multiport PSU, again 5V 2A (or better) thus, to some extent, mimicking what happens in a netbook or laptop etc. Two different supplies can give rise to instability because the "Y cable" tries to "back-power" the Pi (something that some "low-to-medium cost" powered hubs also do). You could also use a single supply and let the "Y cable" back power the Pi with the attendant risk of having bypassed the Pi polyfuse, over-voltage protection etc.
Trev.
** I have yet to find such a hub that costs less than the Pi itself so I prefer to use USB HDD's that can be externally powered w/o the use of a "Y cable".

[RaTTuS]

I have an external hard drive that I would like to connect to the Raspberry Pi. This hard drive has a single USB connector and must be powered through it. It came with a cable that has two USB A type ends to be connected to two USB ports of the same computer, for sufficient power.
The Raspbery Pi cannot give sufficient power to this hard drive, even when using both of its ports.
Question: Is it safe to plug the data end of the USB cable into the Raspberry Pi, and the power-only end into a USB wall charger? Can this cause hardware damage to either the Raspberry Pi or the hard drive?

Not really a "safety" issue but rather one of stability, risk of data loss etc. Whilst the use of a very good powered hub**, powered by a 5V 2A PSU (or better) is one solution another possibility is to power both the Pi and HDD from the same multiport PSU, again 5V 2A (or better) thus, to some extent, mimicking what happens in a netbook or laptop etc. Two different supplies can give rise to instability because the "Y cable" tries to "back-power" the Pi (something that some "low-to-medium cost" powered hubs also do). You could also use a single supply and let the "Y cable" back power the Pi with the attendant risk of having bypassed the Pi polyfuse, over-voltage protection etc.
Trev.
** I have yet to find such a hub that costs less than the Pi itself so I prefer to use USB HDD's that can be externally powered w/o the use of a "Y cable".

see
http://shop.pimoroni.com/products/pihub ;-p

[szhorvat]

** I have yet to find such a hub that costs less than the Pi itself so I prefer to use USB HDD's that can be externally powered w/o the use of a "Y cable".

Thanks for the response. Are you aware of any problems with the 7 port hub mentioned here?

[szhorvat]

Thank you all for the replies. Is the information I can find about "backpowering" also apply to the situation I described (i.e. using the Y cable with a separate power supply)? Or does it only apply to powered USB hubs?

[elatllat]

Likely yes, but a voltmeter will tell you for sure.