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I'm trying to understand just how much speed I'm getting when writing big files out one of my MacBook Pro's USB 3.0 sockets to an external drive, also with USB 3.0. An example is the copying -- via a simple drag of icon in Finder -- of a 0.25GB Photos library.

In theory, for USB 3.0, I guess I could expect speeds of up to 600+MB/s, but probably something below that, maybe to as little as 100MB/s. Ish.

In practice, according to Activity Monitor, "Data written/sec" is about 250MB/sec, but I get something like the same number for "Data read/sec". That's a total of 500MB/s.

So how do I relate theory to practice?

Do Mac copies include a verify? Is that why I'm seeing 250MB/sec for each of writes and reads? And if so, does that mean I'm seeing a pretty respectable 500MB/sec across USB 3.0?

Or am I really seeing only the 250MB/sec reported as "Data written/sec"? That's not disastrous I guess, but quite a bit slower than the 640MB/sec 5Gb/s) theoretical. And if that's what's happening, then what is the 250MB/s of reads that is also being reported? (To be clear -- the writes and reads numbers are similar but not identical, so there actually are two different things being measured.)

  • Use the Blackmagic Disk Speed Test for some objective measurements of all the disk devices on your computer. I've used it to evaluate external HDs for our video editing courses. – IconDaemon Dec 3 '18 at 14:13

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