I have an USB 3.0 2.5'' Drive (without external power source) from verbatim and when I plug it into my old laptop which only has USB 2.0 it is incredibly slow (much slower than my USB 2.0 2.5'' hard drives of comparable size). My guess is that this has something to do with the power, but I haven't ruled out that it might be some other issue, as I'm not sure how to check that. Needless to say, it runs just fine and much faster than my USB 2.0 drive on a USB 3.0 port.
So, my (statistically not significant) experimental result is: It works, but it might be slow, and I'm not sure if it is good for the hard drive to run on too low power.
And about how to find out: I tried to find out about that by looking at the technical details and comparing them, but there was nothing in there that seemed helpful. If I remember correctly, there were other USB drives that explicitly stated that they work on USB 2.0 ports, and mine didn't, so maybe that is something to look out for, although it also might be a case of asbestos free cereal.
Comparing a 7200 RPM 3.5” form factor drive to a 5400 RPM 2.5” form factor drive is like comparing apples to oranges. While a smaller drive might have a smaller RPM, the density of the 2.5” drive’s platters is more than the 3.5” drive. Also, the RPM speed refers to the outer most rim of the disk. Not the core. Meaning most of the time you are using a 3.5” drive, the speeds actually never touch the 7200 RPM and are often in the 5400 RPM range. Not to mention the increased size in a 3.5” drive means seek times across platers is slightly higher than a 2.5” drive.
As far as terminal commands you can us on a Mac to test performance, dd is a great tool for simple benchmarking. So let us assume one drive is mounted as the volume 7200RPM_Drive and the other is 5400RPM_Drive, you could run these tests. First, let’s test the write speed of the 7200RPM_Drive like this:
time dd if=/dev/zero of=/Volumes/7200RPM_Drive/testfile bs=1024k count=2048
And now check the read speed like this:
time dd of=/dev/null if=/Volumes/7200RPM_Drive/testfile bs=1024k
Note the way this dd test works is by creating a file named testfile based on the output of /dev/zero for a write test. And then it reads the same testfile. You get a fairly decent benchmark of speed by doing this.
You can do this with the 5400RPM_Drive like this for the write test:
time dd if=/dev/zero of=/Volumes/5400RPM_Drive/testfile bs=1024k count=2048
And this for the read test:
time dd of=/dev/null if=/Volumes/5400RPM_Drive/testfile bs=1024k
Now when all is said and done, this might give you a new tool to give you the same conclusion: One drive is slower than the other. The only thought I could have then would be the fact you have both of these drives connected to your Mac via USB enclosures. And the Plugable USB 3.0 SuperSpeed SATA III Lay-Flat Hard Drive Docking Station (ASMedia ASM1053E SATA III to USB Chipset, UASP and 6TB+ Drive Support) might not be correctly handling the 7200RPM drive. Meaning the drive is good, but the docking station is not the speed demon it claims to be. I have often seen different mixes of drives behave differently in the same USB enclosure; remember not all bridging circuitry is the same. And if the enclosure is USB 3.0 rated, perhaps the USB 2.0 speed is not great?
Another idea is checking how the USB enclosures are connected. Sometimes of they are on the same USB bus as USB 1.1 devices such as keyboards and mice that could throttle speed down. If there is any way to isolate the connection the USB drive is on that would be the best way to check again.
But when all is said and done when you state the following in the comments:
A lot of people on the 7200-RPM drive's Amazon reviews commented on
how fast it is, but I have found it to be slower than my other drives,
so I'm wondering if the drive I got has a defect.
I’m fairly confident that users commenting on the relative speed of this internal drive are using them with internal, direct SATA connections. And not using a USB enclosure or bridging device. In general, I do not think you received a defective drive. But am fairly confident based on real-world experience that the USB bridging enclosure you have might not be as fast or robust as it claims to be.
Best Answer
The way most vendors work, they do not re-qualify out-of-production hardware for the latest specifications (unless some huge purchase contract relies on it, anyway), so as long as these old models stay in stock, their specs say the same. They may or may not support more modern, higher density hard drives, but vendors and the manufacturer won't guarantee it.
But why 160GB? It's not a common PC BIOS or CHS->LBA limitation, the closest would be 128GB. Back in 2007, 9.5mm drives went to 160GB while 200GB+ you had to buy a 12.5mm drive. That was just the state of the industry at that moment. You can see discussion in Mac forums like this: http://ask.metafilter.com/63097/Whats-the-max-size-harddrive-I-can-put-in-a-MacBook