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macrumors newbie
Original poster
Jan 24, 2020
Hi everyone

I have a 27'' iMac - late 2013 currently running:
- Intel Core i5 4570 3.2 GHz
- 16 GB DDR3
- 1 TB HDD (I can't remember if it's 5400 or 7200 rpm)

Due to slowness I've recently attempted to boot from an external SSD (Samsung T5) using USB 3.0 rather than running off the slow hard drive. I've followed some instructions to duplicate my data to the SSD. And it booted fine to the new drive and things seem to be running quicker.

Here's the problem: I ran Black Magic to test speeds before and after. Before the change I was getting read and write speeds of approximately 140 MB/s. After the change I'm getting around 38-40 MB/s. I checked and confirmed that the USB is running off the USB 3.0 bus and in the System Report, it says the drive max speed should be 480 MB/s.

I've tried running the TRIM command that other posts have suggested but that didn't appear to have a significant impact.

Any suggestions??



macrumors G5
Jul 11, 2009
If you're seeing the maximum speed is 480, that's megabits not megabytes, so your drive is connected as a USB 2.0 device rather than USB 3.0, so the slow speed is about right for USB 2.0.
A device connected as USB 3.0 would show that it's connected at 5Gb/sec (that's gigabits) and then the speed would be substantially faster. Do you have the SSD connected directly to your computer?
Trim cannot be used on USB connected SSDs on Macs, so it would have no effect at all in this case.


macrumors newbie
Original poster
Jan 24, 2020
Oh good point about the USB 2.0 and megabits value.

I was able to figure it out. I unplugged all usb devices - someone on another thread said that if the bus reads a 2.0 device first it affects the rest of the ports. I also reset the PRAM/NVRAM.

Thanks for your help!


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macrumors 68020
Oct 19, 2007
Blue Mountains NSW Australia
Check your Hub is actually a USB3 hub. If it is an older USB2 it will still run at lower speed.

Might be best to upgrade to a USB3 hub. I use a USB3 hub for an exzternal SSD an external Thunderbolt SSD. Whilst they perform at very similar speeds on SuperDuper doing backups, the USB3 SSD is considerably slower at booting than the Thunderbolt.


macrumors Penryn
Feb 20, 2009
Do you (did you) have the t5 connected through a hub?
If so, DISCONNECT it from the hub.
Plug it DIRECTLY into a USB3 port on the back of the Mac.
Reboot and try BlackMagic again.
Does anything change?

In other words... DON'T "boot through a hub"...


macrumors newbie
Original poster
Jan 24, 2020
I had it plugged directly into one of the ports on the iMac in all scenarios (I don't have an external hub or anything)...but seems like the steps I took above have now resolved the issue.


macrumors 6502a
Aug 11, 2017
I had the same issue last night and saw your post suggests doing PRAM reset and it worked! Thanks

The problem happened again after a reboot. PRAM didn't solve it this time.

Well, after another SMC and PRAM reset it turned to 5Gb/s again.
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