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I recently bought a 3tb Seagate Expansion external hard drive. I didn't know that it requires external power supply. I am worried about the fact that the hdd will get damaged if suddenly power goes off. Can I build a usb connector that can supply power to the hdd from by laptop's battery even if the power goes off. It requires 12V-1.5A power supply as written on the adapter. Please tell me if there is any other way.

Thank you.

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Why not using a battery UPS solution. As an example APC - Back-UPS 700VA will give you 62 minutes of battery use. –  Darius Jul 18 '13 at 17:07
    
USB 2.0 nor USB 3.0 can supply 12v. If you are worried use a UPS. –  Ramhound Jul 18 '13 at 17:09
    
@Ramhound Though you could use a DC-DC converter. Still, USB (even USB 3.0) can only provide up to a maximum of 900 mA at ~5VDC per port. 12V 1.5A is 18W, so you'd need four USB 3.0 ports delivering almost full rated power even assuming no conversion losses to be certain that you can power the drive at all times. I'm sure it's possible, electrically, but I doubt that it's practical, particularly with a laptop setup. A small, dedicated UPS is probably a better choice if the worry is unexpected shutdowns. Even the smallest will provide ample power to allow controlled shutdown. –  Michael Kjörling Jul 18 '13 at 17:11
    
@MichaelKjörling - Even if he used a DC-DC converter USB can only supply 4.5W of power with 900MA spread across all ports. –  Ramhound Jul 18 '13 at 17:13
    
@Ramhound Across all ports? Really? At least Wikipedia claims "A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0.". Even that said, I still second the dedicated UPS suggestion, as drawing that much power from USB is going to be flaky at best anyway. –  Michael Kjörling Jul 18 '13 at 17:16

2 Answers 2

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12V 1.5A is equal to 18W, and even if USB can supply that much power in the first place (going by the comments to the question, there seems to be a bit of uncertainty about that) it's going to strain the USB ports quite a bit. USB 3.0 can provide up to 900 mA at approximately 5 V on a single port, which works out to 4.5W of power. In principle it should be possible to use a DC-DC converter to bring this up to 12V, but what you gain in voltage you lose in current. If you could draw the full 900 mA from four USB 3.0 ports, then you would just hit that 18W target, but DC-DC conversion isn't lossless, so in practice you'd need closer to 5-6 USB 3.0 ports all capable of delivering full power when the computer is running on battery to guarantee that you'd keep the drive running. I don't think your laptop has those specs.

Instead, if you are worried about sudden power losses, there are two options which I think deserve serious consideration.

If you were in the position of buying a new drive, you could pick a 1.8" or 2.5" one, since they are generally able to be powered solely from USB, not needing an external power supply. This option is of little use to you since you already have the drive, but may be of interest to others in the future.

That leaves a second source of power, independent from the mains as well as the computer's USB bus. Hence, the suggestion in the comments to get a dedicated UPS to power the external drive through. Even the smallest dedicated UPS should be able to power something drawing a few tens of watts with a yawn, so you could simply pick one based on what's cheapest and probably get something that will power the external drive just fine basically for however long you wish, easily allowing a controlled shutdown.

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Thanks, I think i should buy a UPS, a small one –  Bishal Jul 18 '13 at 18:40

If I were to suggest that you address the root of your concern:

I am worried about the fact that the hdd will get damaged if suddenly power goes off.

Instead of engineering something that is risky at best and will not work at worst with potential warrantee void, is to simply plug the HDD into a power strip that provides surge protection.

Powering on and off the unit is part of it's operation and from experience with two of these units, you are more apt to have heat problems than damage from a power outage. (You can confirm my comment on heat with crystal disk info).

Get a surge protector and use the drive with the intended power supply. Rigging anything different may also void the warrantee.

While a UPS is great for small time frame power interruption (so you can gracefully....Pull the plug?) at $60 a surge protector can be as little as $9.00 and you can plug your other computer equipment into that for surge protection.

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It really depends on what is going on at the time of the power loss. Sudden power loss to the drive while data is being written certainly can result in logical damage to the file system (less of a concern with modern journaling file systems like NTFS or ext3fs, but still certainly a possible cause for concern), or loss of the written data where metadata has not been updated to match causing a journal rollback on recovery (shouldn't really happen, but sometimes you just never know...). –  Michael Kjörling Jul 18 '13 at 20:28

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