I used to use dd on my mac but that's been years ago. dd uses 512k bs by default. I believe for a partition copy they suggest 64 and use noerror
I would try,
dd if=dev/sdd1 of=dev/sdd2 bs=64M conv=noerror
Well, I learned it was pronounced just like judo.
But I learned it from a Solaris guy who was teaching me BSD back in the early 90s, so there's no telling if he knew what he was talking about.