View Full Version : safe to have 2 / more hard drives transferring data connected via a4port usb adapter?

01-04-2017, 04:24 PM
is it safe to have more than 1x external hard drive connected to one of the 4 usb 3.0 ports in my 'mini hub' as pictured? l noticed that there are other 4- ports USB 3.0, eg belkin have one, it comes with a seperate power supply, and it is ~4x the price. won't they just be the same , except my cheaper mini hub draws power from my mbpr when connected?

01-04-2017, 05:41 PM
"Safe" is an interesting term. Yes, it's "safe" in the regard that it can accept 4 drives. It's not "safe" if you plan to power four of them from the MBP as they probably will overdraw from the USB port. Now, if the drives have external power supplies themselves, then it's back to "safe" again because they WON'T be drawing power from the USB port.


01-04-2017, 06:21 PM
So would I end up wiping out the external hard drives attached to this mini port, if I say have 2 attached to this port, simultaneously getting things transferred and 3rd, attached to mbpr USB port directly?
be completely safe if use just one of the 4 USB's for a external hard drive and other 3 for say, charging batteries or smart phones?

01-04-2017, 07:07 PM
Not necessarily, but maybe. The problem is that as soon as you exceed the power ability from the USB port, one or more drives will get powered off, which means that if that drive is in the process of writing something, that write process will fail. IF that write is to the directory, you could lose the entire contents.

Again, the issue is that the total power from the USB port should not exceed the recommended power for the port. IN the case of USB3, if the MBP port is, in fact, USB 3 then the spec for the port. Here is what Wikipedia says about the power from USB3 ports:

Power and charging[edit]
As with earlier versions of USB, USB 3.0 provides power at 5 volts nominal. The available current for low-power (one unit load) SuperSpeed devices is 150 mA, an increase from the 100 mA defined in USB 2.0. For high-power SuperSpeed devices, the limit is six unit loads or 900 mA (4.5 watts), almost twice USB 2.0's 500 mA.[9]:section Power Budgeting
USB 3.0 ports may also implement other USB specifications for increased power, including the USB Battery Charging Specification for up to 1.5 A or 7.5 watts, or the USB Power Delivery specification for up to 100 watts.[11]You can look at the tech specs for your Mac at Apple to see if they define which, if any, of these standards apply. But in general, I would not assume more than the defined 900mA unless I knew with absolute certainty that it could handle more.

Once you know the total capacity of the USB port, you can add the demands from the total set of devices to see if they can all draw at the same time. If not, don't do it.

01-04-2017, 07:37 PM
@ Jake - thanks for your excellent posts on this topic - enjoyed reading!

@ Applet - you really need to purchase a 'powered' USB hub to guarantee that each port is receiving enough voltage/amperage to properly function w/ your attached devices - you can easily search for 'best powered USB hubs' w/ plenty of hits, just one HERE (https://wiki.ezvid.com/best-usb-hubs?id=adw&gclid=CjwKEAiA17LDBRDElqOGq8vR7m8SJAA1AC0_CohIt4IM NobhQpaVCY1Eq-UpSSVk2064Orjfvuqa7xoChtnw_wcB) - be sure to compare the power hub's outputs (which may vary w/ the various ports) to the needs of your devices - I have a few external HDs that did not work well on even a powered hub. Bottom line is to dump your cheap hub and invest some more $$ in one that will work 'safely' for you. Dave :)