This really isn't the correct forum for this post, but I"m not sure where it really ought to be.

Many years ago, someone (V1, I think) posted (this Terminal command)

Code:
sudo dd if=/dev/rdisk0 of=/dev/null conv=noerror bs=1024000

as an alternative way to run a surface scan in the absence of one of the expensive utilities, and I've run it successfully on both HDDs and SSDs any number of times (including on my SSD/macOS 10.14.5 just yesterday).

The need has now arisen (in another thread) to run the command on a fusion drive, but it refuses to run.

Using the original command as a template and this as an "update guide," I adjusted the command to

Code:
sudo dd if=/dev/rdisk1 of=/dev/null conv=noerror bs=1024000

but Terminal returns

Code:
Last login: Wed May 15 11:49:50 on ttys000
iMac:~ myname$ sudo dd if=/dev/rdisk1 of=/dev/null conv=noerror bs=1024000
Password:
dd: /dev/rdisk1: Operation not permitted
iMac:~ myname$

It returns the same error if "disk1" is substituted for "rdisk1".

Has anybody got an idea of why the command won't run on a FD and how it might be adjusted to get it to run?

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