'>>' will cause O_APPEND to be specified as flags when opening "/dev/sda". I'm pretty sure this flag is ignored on block devices as it's obviously useless.

    fd = open("/dev/sda", O_WRONLY|O_CREAT|O_NOCTTY|O_APPEND, 0644);
    pos = lseek(fd, 0, SEEK_CUR);
    -> pos = 0

http://hastebin.com/abuhiwivoz.pl

    # tmp  sudo ./open_sda
    Current position after open: 0
    Current position after seek to end: 128035676160

The redirection will happen in your shell, but the command will be called inside the shell invoked by sudo. So that won't work either unless you have write privs to the block device, but if that were the case, you probably didn't need sudo to read the dmg file. So this will likely fail for a reason other than the one you pointed out.

Correct. In these kinds of situations I usually do

  sudo sh -c 'blah blah blah >> file'

but I don't like it when I have to do that.

A better way to write files as root is to pipe the output into `tee`. eg:

   blah blah blah | sudo tee -a file

This will do the appending as per your example. If you want to write to the file like people are doing with the Ubuntu image then just drop the append (-a) flag:

   cat ubuntu-14.04-desktop-amd64.dmg | sudo tee /dev/sda1

Though obviously the `dd` utility is still a better way of writing disk images than any of the above.

Better:

    cat ubuntu-14.04-desktop-amd64.dmg | sudo tee /dev/sda1

You better redirect tee to /dev/null otherwise your terminal is going to print out that whole file.

Best:

    sudo tee /dev/sda < ubuntu-14.04-desktop-amd64.dmg

I know you meant to use the pipe instead of redirection, but it might be worth updating your comment for the benefit of others who are less command line literate :)

Whoops, fixed.

if you used cat, could potentially something escape and run in your userspace?