conv=sync tells dd to pad each block to the left with nulls, so that if, due to error, the full block cannot be read, the full length of the original data is preserved, even though not all of the data itself can be included in the image. that way you at least know how damaged the data is, which might provide you with forensic clues, and if you can't take an image at all due to bad blocks or whatever, you can't analyze any of the data. some is better than none.
conv=sync,noerror is necessary to prevent dd from stopping on error and performing a dump. conv=sync is largely meaningless without noerror.
http://linuxcommand.org/man_pages/dd1.html
http://vlinux-freak.blogspot.com/2011/01/how-to-use-dd-command.html
