MacOS Sierra: Solving an external drive mounted as read-only after install

When I installed MacOS Sierra, my trusty Drobo, who’d been flawless through so many OS updates, suddenly stopped working. Or, more particularly, suddenly was no longer writable. I just solved this, and, as I was unable to find any of this online, thought I’d write it up in case anyone else has this issue. Particularly as Drobo’s support suggested:

If you’re like me, you have no media capable of holding everything in your Drobo.

Of note: My Drobo is formatted HFS+. If you’ve got an external NTFS or FAT drive, YMMV.

The cause of this change is that in MacOS Sierra, the /Volumes folder is no longer writable by default. So, to change, I needed to do the following:

First remount the drive as writable:

sudo mount -u -w /Volumes/YourDriveName

(the -u means you’re modifying an already-mounted filesystem; -w makes it writeable).

I could now write to it if I sudo vi a text file. But — neither TimeMachine nor (most importantly for me) Lightroom could write to it at this point. It turns out that ownership of everything in the Volumes folder changed too, away from root:wheel. So, next step:

sudo chown -R root:wheel /Volumes/YourDriveName

(the -R is recursive, so all files. root:wheel is the owner & group — you could make this your own user, but I chose to make it match other “system” files here.

Lastly, check the read/write privileges. For me, everything in the Drobo was now -r--r--r-- (read only). Which just won’t do at all. I wanted everyone to read, and owner and group to be able to write. So, next:

sudo chmod -R 774

This changes permission so that the owner & the group can read, write, execute, and everyone else can read — which is what I wanted for this drive (in particular, group-writable was important for Lightroom, while TimeMachine seemed perfectly happy to start working with only the owner having write permissions).

So — this may have been a particular-to-me issue, but given that I have many, many TB of personal photos, videos — and TimeMachine backups on my Drobo, I’m certainly glad I took the time to solve this.

What’s left to do

So, I can read and write to my drive — but I still can’t figure out how to get the drive to show up in the list of Devices in the finder. I can live with this, but it would be nicer if it just “appeared”. If you know how to solve that (and no, clicking the “show external drive” off and on in preferences doesn’t work), let me know!