Can USB 2.0 port power a USB 3.0 external HDD?

yes but make sure you have the power adapter plugged in when using it. some external devices do not work when running on battery power

as i said..."some" laptops. not "all" laptops.

Assuming you haven't as yet purchased that Transcend USBEHD, let me ask you something...

1. Is portability of the USB external device important or even critical for you? Do you plan to transport the external drive more or less routinely? Or will it, for the most part, simply be stationary at the location of your PC or PCs and only occasionally transported from your site?

2. Assuming that portability of a USB external HDD IS NOT A CRITICAL FEATURE OR REQUIREMENT for you...

3. Think hard & long before you invest in this product or a similar type product.

4. In my opinion, you, as well as most PC users, would be much better served by purchasing a USB external enclosure + a HDD/SSD for their backup/storage strategies rather than a commercial "one-piece" unit such as the Transcend unit you're considering. For one thing this USBEHD is NOT equipped with a power adapter; its operation depends on power supplied through the USB port it's connected to. Now that in itself is not the real problem here; your USB 2.0 port should be able to provide ample power to the enclosed drive, which of course will be a 2.5" HDD probably a 5400 RPM model (although I'm not certain of its RPM).

5. The advantages of purchasing separate components of your choosing, i.e., the USB enclosure and the HDD (or SSD if you prefer) are significant advantages. First of all, virtually any USBEHD enclosure you purchase will accommodate both 2 1/2" & 3 1/2" drives. That in itself is a real advantage. Then you can purchase the HDD (or SSD) YOU want, not the HDD that's usually installed in the commercial product. Rest assured the drive YOU purchase will invariably prove superior to the commercial product. And at the outset you can select whatever disk capacity YOU want.

6. Another significant advantage is that your HDD (or SSD) that you install in a USB external enclosure can always be easily removed and installed in a PC should that need arise. Keep in mind that many of the commercial "one-piece" USB external drives are equipped with a non-standard SATA data connector on the enclosed SATA drive and as a consequence cannot be connected to a standard SATA data connector of a HDD or SSD should that need arise. (The Transcend external HDD unit may be one of those; I don't know).

7. Still another significant advantage is the fact that by using a separate USBEHD enclosure + HDD (or SSD) you will have UNLIMITED capability to utilize any number of different HDDs/SSDs for whatever purpose(s) you deem necessary or desirable. So while you may initially want to purchase a 4 TB HDD, perhaps at some later date you will want to purchase one or more additional HDDs (or SSDs) that can be utilized in you exisiting USB external enclosure. So you achieve an enormous degree of flexibility by purchasing the separate components referred to.

8. All in all there are really no significant (if any!) downsides to purchasing a USBEHD + HDD (or SSD) in comparison to purchasing a commercial one-piece product. Certainly not in performance and neither in cost. And you will have an enormous choice as to the type & particular model USB enclosure that appeals to you.

9. There are an enormous number of different USB external enclosures/"docking stations" available from the online vendors, e.g., Newegg & Amazon. You would do well to peruse their wares. These USB external enclosures of one type or another are frequently on sale and generally obtainable at modest prices.

10. And it's simplicity itself to install and/or reinstall a HDD or SSD in & out of a USB external enclosure - whatever the type.

11. You might also want to consider one of those SATA-to-USB adapter device for connecting a SATA HDD or SSD to a USB port. Here's a sample of that type of device...
http://www.newegg.com/Product/ProductList.aspx?Submit=E...
(By & large they haven't been my favorite type of "USB external device" since over the years in general we've found them erratic in operation.) But for the past year (based on recommendations we rec'd) we've been using this Anker SATA-to-USB adapter device (see https://www.amazon.com/Anker-Portable-Adapter-Cable-Pow... and it's been performing flawlessly with a wide variety of 2.5" & 3.5" disks)

12. Then there's the so-called "docking stations". They're fine if one likes that style - (we have three of them hanging around that are rarely used). My chief problem with them is their bulk although I have noticed the newer models are getting more compact in size. It's really a matter of user preference. By & large they all do the job. And virtually all of them accommodate both 2.5" & 3.5" drives.
See for example http://www.newegg.com/Product/ProductList.aspx?Submit=E...
and there are literally scores of other models you can peruse.

(My links may not be up-to-date but I think you get the picture, right?)

In any event do a bit more research before making a final decision, OK?

in that case you should look at getting a western digital passport. they are portable and are powered by the USB port

https://www.newegg.com/Product/Product.aspx?Item=N82E16822235145

Of course it's your choice but I trust you do undertand that if you install a 2.5" HDD or SSD in a USB external enclosure (in virtually every case) you will not need a separate A/C power adapter for that device. Your laptop's USB 2.0 power (in virtually every case) will be sufficient to power the 2.5" HDD or SSD.

So the bottom line here is that you could still employ a USB external enclosure that actually will accommodate 3.5" drives as well (in case they're ever needed in your environment), but utilize the USB external enclosure for your 2.5" drives without the need of an A/C power adapter.

You do understand all this, yes?

But if you're SOLELY interested in using ONLY 2.5" HDDs (or possibly SSDs down the road), why not pick up something like this?...https://www.newegg.com/Product/Product.aspx?Item=0VN-0003-000T8

We have a couple (maybe three) of these Orico enclosures around here in black & white. They work just fine. They're "tool-less" so you can easily change 2.5" drives whenever the user may want.

But as the old politician once said, "Yer pays yer muny an' yer maikes yer cherce".

That recommended western digital seems a good deal at $120 - kinda ugly looking case though - imho - but who cares about that?

If 4 TB is a bit too small for your needs, here's a 5 TB for $140 that runs off of usb 2.

https://www.newegg.com/Product/Product.aspx?Item=N82E16822179115&ignorebbr=1

Oh ... and thinking of that StoreJet drive you're thinking of buying ...it's hard to be certain but it looks to me like it would work fine plugged into a single usb 2.0 port.

Here's something I heard about laptop usb ports (it may well be an urban myth). Apparently many laptops have 2 ways of connecting the ports to the mbo - some are directly soldered to the board and some are connected via a ribbon cable. Rumour has it, the ones that are directly soldered to the board have a little more kick to them then the ribbon connected ones. So ... if you have a choice... maybe use a directly soldered port for your external hd and use a ribbon cabled one for something like an external mouse.

AHA! I'm afraid there is some mis-information in answers above, but you have found a good solution in the Seagate Expansion unit, OP.

USB2 ports can supply up to 0.5 A current at 5 VDC to connected devices. USB3 raised that limit to 0.9 A.

My understanding is that VERY few HDD units - even 2.5" low-power ones used in laptops - can run on 0.5 A. In fact, it you review the older "Portable Hard Drives" sold for USB2 systems you will find two dominant designs regarding power. One includes a separate power adapter that you must plug into the external drive to get it to work. That is what you have already in the 1 TB unit you mention. The other was a special power cable that came with many of those "Portable Drives" that had TWO USB2 connectors on its end. You had to plug BOTH of those into separate host USB2 ports to get enough power to run the unit. In fact, some people have found that getting one of those old cords can allow users of the new USB3 Portable Drives to use them (at USB2 speeds, since that's the cable they are using) with computers that have only USB2 ports, but you still need to use TWO ports.

The USB3 system solved the power limit problem for "Portable Hard Drives". The new ones on the market use 2.5" HDD's designed for Laptops with several features to minimize their power requirements, and CAN operate perfectly well within the 0.9 A limit. The Transcend unit you cited first is a good example of that. But typically when you connect them to a single USB2 port (no special double-headed cable) they start out looking like they are working, but never actually succeed in reading or writing anything. They just return errors to the computer because they malfunction with too little power. I have never seen any "USB3 Portable Hard Drive" that can work on a standard USB2 port with no additional power supplied. I consider it unfortunate that the promotional material for all these devices fails to tell you this. The tell you they are "USB2 compatible", but don't tell you you have to figure out how to provide adequate power to get it to work.

For someone like you, OP who has only one USB2 port available on the host computer, the only way to get a USB3 Portable Hard Drive to work is to arrange for more power. One way, as I said, is to scrounge up one of those double-headed old USB2 cables and use that IF your computer has two ports. Another is to do what you have done, OP - find a USB3 Portable Hard drive that includes its own power "brick" that you MUST connect when using a USB2 port. The third way is what ArtPog was getting at. Rather than buy a ready-to-use Portable Hard Drive, buy an external enclosure with a USB3 interface (and often with additional optional interfaces) that comes with its own "power brick" (since you need one anyway) and install in it a HDD of your choosing. If you get a slightly large one, you will not be restricted to using only the small (and expensive) low-power (and performance) 2.5" laptop HDD's inside. You could use a higher-performance 3.5" HDD, which FOR SURE will need more than 0.5 A and more than 0.9 A and have a more powerful "Portable Hard Drive" system with higher capacity at a similar price to the small "Portable" units.

We had to deal with exactly this problem. We bought a Transcend Portable Hard Drive that was sold as a USB3 unit (but we did not realize that). It could not work on a USB2 port, even though it sort of looked like it was trying. I found a different solution for use with a laptop that had only USB2 ports. I bought a powered USB3 Hub - one with several USB3 ports that came with its own "Power Brick" sufficient to power all the ports of the Hub. Then any USB2 or USB3 cable to the laptop's USB2 port provided the data connection between Hub (and its connected Transcend drive), while the power for the drive was provided by the Hub's own power source. It works perfectly that way, but at USB2 data transfer speeds, of course.