How to mount an external drive read only on the first try 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:  


mount all disks read only or 
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 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





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





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.