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I'm trying to decide whether to replace an existing hard drive on an iMac 10,1 27inch.

I'd much prefer to replace this drive with an external drive as I can get a tremendously bigger drive and the portability of having the OS external.

What I don't understand is that I keep reading that USB 2.0 is very slow in reading and writing files, but none of the articles I've found specifies whether this applies to the computer USB port, the external drive port, or both.

The iMac has 4 USB 2.0 ports, but the external drive uses USB 3.0.

I'm assuming that both ports need to be 3.0 but I can't find any verification on this. It doesn't need to be super fast, but I don't want it to be super slow either.

So my question is, can I effectively run an iMac booting from an eternal drive when the computer runs USB 2.0 and the external drive runs USB 3.0?

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    Short answer, not efficiently. It will work, but it will be molasses. You can't force a USB2 post to be USB3. Get the largest drive you can & use it internally. Those iMacs take standard 3.5" drives - now available up to 8TB.
    – Tetsujin
    Commented Jun 23, 2015 at 6:50
  • Yeah it seems to be the consensus. Financially I'm relegated to a 2TB internal or Firewire external. Thanks for the help Tetsujin.
    – secondman
    Commented Jun 23, 2015 at 11:03
  • I guess the only question left would be, is there a difference in the external drive being a powered device versus just a USB device? All the drives I'm looking at are AC powered, not just plug and play USB devices.
    – secondman
    Commented Jun 23, 2015 at 11:07
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    Nope, still gonna be slow as all hell ;)
    – Tetsujin
    Commented Jun 23, 2015 at 11:11

2 Answers 2

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A brief education on USB 2 vs 3:

Think of USB as a pipe (It's actually called a "bus"....Universal Serial Bus) that can only push so much data through it per second. For USB 2, this limit is 480Mbps, or 480 Megabits per second, and for USB 3, it's 5Gbps...5 Gigabits per second. This means that across ALL USB 2 devices connected to your computer, the bus will at most support 480Mbps of throughput. Put another way, if you have 2 devices rated at 400Mbps connected at the same time, you will max out the USB 2 capability before you max out the device capability.

USB 3 is backwards compatible with USB 2. This means that your USB 3 drive WILL work when plugged into a USB 2 port. It will only operate at USB 2 speeds, because that is the bottleneck.

Now, to actually answer your question: As stated above, most USB 3 devices work in USB 2 ports. It will just be very slow. Your assumption that they both need to be USB 3 is incorrect.

As for whether you can run your machine from an external drive: yes, you can. You don't want to though. It will be VERY slow. To give you an idea, at work, we have some Seagate drives that are god-awful terribad slow, and those clock in at around 71MBps (capital "B" here; I'm talking in bytes not bits) read speed and 69MBps write speed. USB 2, however, only supports 60MBps (480 megabits / 8 bits per byte = 60 megabytes). Even these hard drives at work would slow down even more.

Now, think what it would be like if you wanted to use ANOTHER drive as a time machine backup. You just cut those speeds in half because the bus needs to be shared. Ouch!

As another point of interest, you expressed an interest in having your OS mobile. In general, this is not a fantastic idea, ESPECIALLY with a spinning drive. If you ABSOLUTELY need a mobile OS, I suggest you find yourself a 64GB flash drive on sale (Newegg had one for 18 bucks just the other day, with a lifetime warranty) and put your OS on that. Then, use a normal internal drive for all of your data. Then, back up your data to some OTHER external drive, or whatever, if you need to bring it with you.

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USB 2.0 is USB 2.0.
It won't get any faster, no matter what you connect to the other end.

And it will be too slow to be really usable. Believe me, I tried that for laughs once and it was even worse than I expected.

Firewire is better for speed (not by much, but at least you get consistent throughput, which USB won't do), but way more expensive than buying a new internal drive.

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