dd is awful and error-prone for this sort of use.

Use noerror, but forget sync? Corrupt output file if there is an error. Use a bigger bs so it's not slow as treacle? A single faulty sector blows away a whole bs of data, and your output image may get unwanted padding appended to the end. Recoverable error? dd's not going to retry.

Use ddrescue or FreeBSD's recoverdisk(1). They're faster, they're safer, they're more effective, and they're easier to use.