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Comment by akdev1l | Hacker News Reader

    cp file.iso /dev/sda

This isn't portable though, it's a GNU-specific "smart" things. Elsewhere, this overwrites the target device file.

Redirection doesn't overwrite device files, so

  cat foo.img > /dev/sda

works and should be portable. No control over the buffer size, though.

And no printing of progress either.

How exactly is cp not portable?

Whatever behaviour you’re describing is not how POSIX mandates cp to work. In fact you can do this with busybox too.

cp will open("/dev/sda", O_WRONLY | O_TRUNC) and simply start writing to that file descriptor.

this works great but in my experience sometimes you should add

  && sync

not sure why it's necessary only sometimes!

The kernel has a cache used for block devices.

If you want to be more targeted:

    blockdev --flushbufs /dev/sda