MacPorts and /opt/local on Catalina and Big Sur read only volumes

Tabitha McNerney tabithamc at
Sat Jun 26 03:28:10 UTC 2021

Maybe the pkg installer software looks to see if an installation is trying
to mkdir in root and it automatically creates /etc/synthetic.conf with an
entry. That would be quintessential Apple to do something like that. Thank
you Apple, it just works.

On Fri, Jun 25, 2021 at 5:20 PM Ryan Schmidt <ryandesign at>

> On Jun 25, 2021, at 21:03, Tabitha McNerney wrote:
> > On a fresh Catalina or Big Sure system, if you cd to root / then sudo
> then try mkdir /opt or something else such as mkdir /hello the system won't
> allow it, I get this:
> >
> > mkdir: /hello: Read-only file system
> >
> > note: the MacBook I just tried this on also has FileVault enabled and
> its got one of those Apple T2 chips with a touch bar
> >
> > How does the MacPorts Catalina or Big Sur pkg installer work around this
> restriction?
> I don't know that we're doing anything. It just works.
> There is some mechanism, that I don't understand, by which the system
> volume and the data volume are combined into a single presentation.
> Presumably the Installer app knows that you cannot install to the system
> volume, so it installs MacPorts to the data volume.
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