What is the best way to monitor the wear of my new external USB boot SSD? I'd like to order a new one to have ready before it gets too slow. I have WD Blue and love the speed, and it worked perfectly the first time.

Also, I turned on TRIM via "sudo trimforce enable". Was that a good idea with WD Blue?


  • Does there exist a USB SSD that supports TRIM? Feb 25, 2021 at 22:51
  • @DavidAnderson My understanding of the issue is that it is not the SSD, but rather that macOS does not support the USB protocols required for TRIM to work with USB drives. But, I agree that trying to enable TRIM on USB is a waste of time.
    – Gilby
    Feb 26, 2021 at 1:14
  • My WD Blue SSD supports TRIM, but perhaps only at the SATA level (I have the USB cable conversion). So maybe running "sudo trimforce enable" was a no-op. Apple added trimforce for external drives in Yosemite.
    – vonlost
    Feb 26, 2021 at 1:27
  • @Gilby and vonlost: My understanding is that, without TRIM, a drive will eventually slow down. However, modern SSDs are so fast compared to the up to 10 Gb/sec USB speeds that even if the drive slows down, the drive is still faster than the data can be transfer over USB. Therefore, supporting TRIM over USB is not necessary. Feb 26, 2021 at 2:09
  • @DavidAnderson Many manufacturers say that TRIM is unnecessary or even bad for their drives as the equivalent cleaning and rearranging data is done by the drive. Samsung recommends against, don't know about WD. The horrors of drives slowing without TRIM does not apply to most (all?) recent drives (but I don't have a reference for that).
    – Gilby
    Feb 26, 2021 at 7:00

1 Answer 1


If your external disk interface supports it, a good tool is to use smartctl. You can get it by installing HomeBrew and then running this command in the Terminal:

brew install smartmontools

Then run smartctl like this:

smartctl -a /dev/diskX

where X should be replaced with the disk number of the SSD in question.

In the output, you want to look for a line like this:

Percentage Used: 1%

This is an indication of the wear on the drive. However, note that it is not an exact science - the drive could wear out before it reaches 100%, and it might also reach values above 100%.

Note: Wearing the drive down doesn't make it slow, as you seem to indicate in your question. When the drive wears out it will fail to be able to read some or all of the data that you have stored on the drive.

Make sure you have a working backup system in place. Just looking at the wear indicator and replacing the drive when the indicator is high won't ensure that you will keep your data.

  • How do you make that work with macOS 11? For external drives, I have had to eject the drive and reconnect before macOS will load the required kext - a bit hard to do with a boot drive.
    – Gilby
    Feb 26, 2021 at 1:10
  • Thanks, I'll check out home-brew/smartctl.
    – vonlost
    Feb 26, 2021 at 1:31
  • Making it work? I initialized the SSD with DiskUtil, did a full install of 11.2.1, it rebooted automatically to the SSD, then a Migrate Assistant and it just worked. I can set the Startup Disk as I like.
    – vonlost
    Feb 26, 2021 at 1:41
  • I mentioned the slowing because of a lecture by Ari Harari where he talked about the physics, how writing gets slower and slower nearing 100,000 operations. Perhaps the controller "absorbs" this delay for users, as much as it can.
    – vonlost
    Feb 26, 2021 at 1:46
  • 1
    Bummer, I get Smartctl's "Operation not supported by device" for both WD Blue SSD USB and Seagate HD USB. It works for internal Macintosh HD. I'll have a look at "kasbert/OS-X-SAT-SMART-Driver".
    – vonlost
    Feb 26, 2021 at 2:23

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