GNU shred takes quite some time to write over a 500GB HD. Can I pause/abort/resume? I have specified that it should be re-written over 3 times, but can I quit (Ctrl-C ?) and call it a day?
If you want to, you can quit after for example one iteration. That is not secure (as in it's impossible to restore anything), but normal tools available for everyone can't recover files then. See for example this page for some more information.
You can pause it at least with sending command SIGSTOP, assuming you know the process id (PID):
You can resume with
Or you can use ctrlz to pause and command "fg" to resume. Please note that this pause/resume do not allow for example unplugging disk or rebooting in between, as it is not closing file descriptors.