How do you setup the Mac to mount the next disk you connect (through USB for example) to mount read only?

My goal is to access a disk without making any changes to it so mounting it read/writing and then changing it to read only does not work.

I can imagine two forms of solution to this question:

  1. mount all disks read only or

  2. don't automatically mount disks that are connected. In the 2nd solution you could then use a terminal command to mount it read only.

  • I assume you are talking about disks you‘ve never accessed before (so you wouldn‘t know any Device UIDs or similar)? – nohillside Nov 20 '18 at 19:02
  • What needs clarification in your question is the next disk you connect by next you mean for first time or any disk ? so you can not use existing UID in Terminal. – Ruskes Nov 20 '18 at 19:50
  • I am talking about a new disk that has never been connected to this machine before. If it had been connected then I could get the UUID and edit fstab to tell it to mount that disk read only. – Rumboogy Nov 22 '18 at 6:22

I use a free app called Disk Arbitrator to ensure I mount any disk I want to mount as read-only.

It's a menubar app and has a Disks Window that you can also interface with.

Note that the Xcode Project files are available as well as precompiled binaries, if you do not have Xcode or want to compile it yourself.

Here are some screenshots:

The Disk Arbitrator icon is the one on the far left and is showing in red for when it's set as read-only.


Showing the Set Mode sub-menu for: Mount read-only

Disk Arbitrator menu

One of the Disks Window showing a thumb drive mounted as: Read-only

Disk Arbitrator Disks Window

Note that I am not affiliated with the author of the Disk Arbitrator project and am just a user that's happy to have an app to handle this operation when needed/wanted.

| improve this answer | |
  • (1 of 2) I tested this out and I did not get the expected overall result. I am not sure where the problem lies but I will share the details since others may benefit. My goal was to remove one HDD from a hardware RAID 1 box and mount it read only in a newer enclosure. The purpose of doing this is that the newer enclosure is much faster and I wanted to copy all the data off of the HDD. So I installed and ran Disk Arbitrator before connecting my HDD and sure enough mounted the partition as read-only. (continued below) – Rumboogy Nov 22 '18 at 20:49
  • (2 of 2) Once I was done copying data off I put the HDD back into the RAID 1 box and powered it on. It then started to do a RAID rebuild meaning that somehow the box detected that the HDD had been removed. At this point I am not clear how it could have detected this but in any event it happened. This Disk Arbitrator program seemed to do what what I wanted so I would say that it looks like a viable solution to mounting disks in read-only. – Rumboogy Nov 22 '18 at 20:49

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