I have a host of USB 2.0 external HDDs that I'd like to plug into a single hub. Usually I don't actually access more than one at once, but you never know. Basically, I'm not sure how USB hubs work with the USB spec. If I buy a USB 2.0 hub, and transfer data off two drives at once, will I get my speed halved? Would buying a USB 3.0 hub fix it? Is there any other advantage to the USB 3.0 hub? (all drives have external power)
USB 3.0 gives you 900ma vs 500ma of power. If you copy data from 2 usb devices the same time to your hard drive they WILL have to share the 25-30mb/s that USB 2.0 has available. The only way you benefit from USB 3.0 is if you have a USB 3.0 port on your computer and a USB 3.0 hub all devices will get a dramatic speed boost otherwise no. Even then the USB 2.0 devices plugged into the 3.0 hub will be limited to 25-30mb/s each. Depending on the hub you might be able to have 2 or more USB 2.0 devices transfer 25-30mb/s at the same time.
I did get a boost in transfer speeds when using USB3 hub and port with single USB2 hard disk. That said you obviously shoud have have the USB3 port in your computer to connect the hub to. It seems that in practice there are differences between hubs even when theoretical port speeds would not suggest it. External WD USB2 disk connected straight to my laptop USB2 port gave me approximately 25MB/s read and 20 MB/s sequential write speeds. Same disk connected to laptop dock USB2 port gave 20MB/s read and 16MB/s write. Again same disk with USB3 hub connected to laptop USB3 port gave 31.4 MB/s read and 28.9 MB/s write. Speeds were measured with CrystalDiskMark.