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Trying to copy a 1TB SDD to a file using dd (iMac recovery mode, to USB3 external HDD). I've read a hundred studies of what is the best bs to use, and the differences between using /dev/diskX and /dev/rdiskX.

I've tried both the 'raw' disk, and the 'normal' disk, and varying bs sizes. Leaving it overnight to see how it gets on, and everything is dead slow. This can't be complicated right? This is the lowest level of data transfer, I should surely get a reasonable level of data transfer?

The best I've got is about 200GB after leaving it for 30 hours. Why?

user4893295
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  • Did you chech the hardware (HDD, USB-SATA-bridge, cable) and the loaded kernel (ohci/xhci/ehci) for being up to delivering speed? I had configurations where a USB3 port was being driven in USB1 mode. Also, fake SSD which would only achieve 10 MBps write speed. – Hermann Nov 20 '21 at 01:54
  • It's an iMac 27" 2015, with a Western Digital USB3 HDD connected straight into it via the USB3 cable it came with. The iMac is booted into recovery mode, having been upgraded to Monterey. So (in my mind at least!) it's pretty much as good as it gets, surely? – user4893295 Nov 20 '21 at 02:10
  • A couple of Q&A similar to yours are available here, and here – Seamus Nov 20 '21 at 04:09
  • Thanks - I have tried a variety of bs sizes (and cat - didn't realise I could do that!). The fastest I can get is 308KB/s. I've checked and the external HDD says "up to 5Gb/s" in system report so definitely connected via USB3. Just don't get it :( – user4893295 Nov 21 '21 at 10:37

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