if - infile
of - outfile

Everything is file in UNIX/Linux, dd works with raw devices, no mounting required. Please check out 'man dd'.

Sorry, I don't understand your answer. My question was.. Let's say my external drive is called "backup" and is mounted on my desktop, would the code be..

dd if=/dev/disk2 of=backup bs=1M conv=noerror

ie.. how does it know which drive to copy the image to?

Please assume I'm an idiot and need things explaining as cleary as possible.

Thanks. I really appreciate it

no, you are not an idiot. You just need to learn how it works.
No, dd has no any idea which drive to use, it is the human being who can specify it.
devices can be found under /dev, disks (or lookalikes) can be found under /dev/disks. You need to identify your external disk.
sudo fdisk -l can be used to list all the available disks and partitions. Also you need to know if you want to use a device or only a partition on it, or just a file. dd does not really care about that and will destroy anything on the device you specified as of=.

.del.

Ok so I've identified the disks but I'm getting an 'illegal numeric value' error when trying to run the code. See attached screen shot. I want to clone the entire contents of disk3 onto disk2.

Where am I going wrong?

Attached Thumbnails

 

of=/dev/something, do not forget /dev.
bs=1M looks incorrect. Which version of dd is it? Which OS is it?

I've tried this line but still get the illegal numeric value error..

dd if=/dev/disk3 of=/dev/disk2 bs=1M conv=noerror

I'm using Terminal on Mac OS High Sierra. I have no idea which version of dd it is.

I'm afraid I'm clueless on the whole process. I'm literally just following instructions given to me earlier in the thread. If there's another way of achieving this I'm happy to listen.

Is dd something that actually needs to be installed on my machine?

dd is installed and working (but in general yes, it should be installed).
You can check the version (in general) like: dd --version. You can use bs=1000k or check man page (man dd) what is excepted as block size.

Sorry, I really appreciate your help but I don't understand what you mean.

Is anyone able to supply a line of code that I can just copy and paste?

Ok I've worked out what you mean by check man page. The section on bs as follows..

bs=n Set both input and output block size to n bytes, superseding the
ibs and obs operands. If no conversion values other than
noerror, notrunc or sync are specified, then each input block is
copied to the output as a single block without any aggregation
of short blocks.

Appeared to finally be getting somewhere with this..

sudo dd if=/dev/disk3 of=/dev/disk2 bs=1024 count=1024

But then I got a pop up saying The Disk You Inserted Was Not Readable By The Computer.

I'm assuming this just isn't gonna work?

probably /dev/disk3 is not valid. or /dev/disk2. I mean this is not the device you wanted to use, but something else, probably nothing. You can check by:
ls -l /dev/disk2 /dev/disk3 probably

To create an image file of disk3 onto disk2, (you may need to be the root user).
That should put a roughly 40GB image file onto disk2, however, it won't, because of the file system on that disk, largest file size allowed, I think, is 4GB.

If you use dd to clone disk3 onto disk2, you will end up with a 40GB disk imaged to it.
As you are on a Mac, or at least it looks like you are, you may have to use /dev/disk3s1 &/or /dev/disk2s1 - /dev/disk3 is 'ntfs' whilst /dev/disk2 is 'fat'.

Edit: It may be easier to write the disk image file to your hard drive, & then, if you want to put it somewhere else, you will need a file system that supports large (40GB) files.

P.S. I notice in your original post you were looking to create an image of a Linux drive, neither of these two drives has a Linux file system.

Don't use a block size and use sync at end.

Be sure to read your dd manual for it's use. Your image stated that BS= was a bad command.

Right.. I've finally managed to clone the drive! (I ended up using a virtualisation of Linux on the Mac) BUT... when I plug the new cloned drive into a new unit, nothing is showing up on my network. The only thing that's different is my original drive was an IDE. My new drive is a SATA. But surely that'd have nothing to do with it??