I have been reading and writing the same files on two external hard drives connected to the same computer, a 2011 MacBook Pro. I was expecting the 7200-rpm EHD to be faster than my 5400-rpm EHD, but that was not the case.

The 7200 RPM, 64 MB cache drive is this HGST model.

  • It’s an internal drive connected to my Mac with a USB 3.0 docking station that has its own power source.
  • 4 TB capacity, less than half is used.
  • 3.5 inch disk
  • Format: ExFAT

The 5400 RPM, 8 MB cache drive is this portable WD drive.

  • It’s a portable drive without an external power source.
  • USB 3.0
  • 1 TB capacity, more than 3/4 full
  • 2.5-inch disk
  • Format: Mac OS Extended (Case-sensitive, Journaled)

For opening a large directory, the 7200 RPM drive can be 1-5 seconds slower. For writing files, the 7200 RPM is even slower, especially with large files.

Even though they both support USB 3.0, my Mac only has USB 2 speeds on both USB slots.

Is there anything wrong with my 7200-rpm EHD that it’s making it so slow? Are there commands I can run on the Mac terminal to get more info of these drives? How can I perform checks on these drives if necessary?

  • Are you expecting one drive to be faster than another just because it spins faster? Nov 2, 2014 at 8:24
  • 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.
    – randwa1k
    Nov 2, 2014 at 20:23
  • Why not do a fair comparison and reformat your new drive as "Mac OS Extended"?
    – Jason
    Nov 21, 2014 at 0:00
  • A higher spindle speed (RPM) does not always equate to a faster data transfer speed. A faster spindle speed means that the access times are usually lower than a similar model with a slower spindle speed. Nov 21, 2014 at 1:34
  • @randwa1k: See this question on Ask Different: apple.stackexchange.com/questions/56872/…. ExFAT is specifically designed for flash memory not mechanical hard drives so you should try using another file system like HFS+ (or FAT32 if cross platform compatibility is needed) to see if you get better results.
    – James P
    Nov 21, 2014 at 11:44

4 Answers 4


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.

  • 3
    “Also, the RPM speed refers to the outer most rim of the disk. Not the core.” — This is not correct. RPM stands for revolutions per minute and measures angular speed independent of size. (Given two disks with the same speed in RPM and different platter size, the larger one has additional outer area which moves faster (in linear speed) to keep up with the middle.)
    – Kevin Reid
    Nov 21, 2014 at 15:24

Just because the drive has better statistics does not mean it is faster. There are many things to consider. The biggest issue I see is that they are both hooked up to USB 2.0, thus making a lot of the speed advantages of the 7200rpm drive irrelevant. If the drive was hooked up internally to a SATA III bus then you would see a difference.

Per wikipedia:

USB 2.0 was released in April 2000 (now called "Hi-Speed"), adding higher maximum signaling rate of 480 Mbit/s (due to bus access constraints the effective throughput is limited to 35 MB/s or 280 Mbit/s), in addition to the "USB 1.x Full Speed" signaling rate of 12 Mbit/s.

Per Sandisk:

Difference between SATA I, SATA II and SATA III What is the difference between SATA I, SATA II and SATA III? SATA I (revision 1.x) interface, formally known as SATA 1.5Gb/s, is the first generation SATA interface running at 1.5 Gb/s. The bandwidth throughput, which is supported by the interface, is up to 150MB/s.

SATA II (revision 2.x) interface, formally known as SATA 3Gb/s, is a second generation SATA interface running at 3.0 Gb/s. The bandwidth throughput, which is supported by the interface, is up to 300MB/s.

SATA III (revision 3.x) interface, formally known as SATA 6Gb/s, is a third generation SATA interface running at 6.0Gb/s. The bandwidth throughput, which is supported by the interface, is up to 600MB/s. This interface is backwards compatible with SATA 3 Gb/s interface.

SATA II specifications provide backward compatibility to function on SATA I ports. SATA III specifications provide backward compatibility to function on SATA I and SATA II ports. However, the maximum speed of the drive will be slower due to the lower speed limitations of the port.

Based upon those stats the fastest the 7200rpm drive can transfer data is somewhere near SATA II speeds.

Regardless the maximum speed they can both transfer is 480mbps because of the USB 2.0 limitation. Then when you consider the drive is 4x bigger than the 5400 the seek time will be slightly longer, add fragmentation, cluster size, disk format, etc and you will locate the source of the slower speeds.

It is like buying a Ferrari and putting a VW Bug engine in it. In order to get maximum speeds all parts must be matched and the hardware needs similar specs.


Just the drive sizes alone should tell you something.

For the 5400rpm disk to find any data it has only has to seek thru 1TB worth of space. For the 7200rpm it has to seek through 4TB. There are simply more sectors to seek though to find anything on the 4TB drive than on the 1TB. (Not that it does seek the whole drive but the point is more space = more possibilities of address).

What are the respective cluster sizes of each of the disks?

If you have gone with the default in formatting the 7200rpm disk then it's likely that you've got 4096 byte clusters. You say things get slower with larger files, this could be because you have used small cluster sizes like 4K instead of 64K thus dividing the disk into more blocks so the larger files gets fragmented across the smaller sectors more easily which in turn means the seek time is (perhaps) significantly increased.

The only real way to make a comparison (if one can really be made at all) is to scrub both disks, and reformat them same size clusters, then load in exactly the same files on to both and defrag both disks, and then do your testing.

At the moment however as others have said you are comparing a variety of fruits as if they were all bananas.


ExFAT is optimized for flash drives, so even though the disk drives are different (USB 2.0 mostly shades this), the bigger issue here is that ExFAT is simply a slow file system to use in a magnetic drive. You are not the first one to notice it: Why is exFAT so slow on Mac?

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