May not work, OS dependent. According to POSIX "sync" flushes the file system level buffer cache of opened files, and not necessarily the block device cache inside the kernel. (Eg. "sync" flushes devfs under /dev, so updates the /dev/sda directory entry, but maybe not the block cache of the (8,0) device.)

There's a good reason why the dd command exists, believe me.

FYI: https://linux.die.net/man/2/sync syscall is not the same as https://linux.die.net/man/2/fsync syscall.

ddrescue worked for me better than dd.