macOS Catalina 10.15.x auto_master resets after upgrade, breaking automounts

After upgrading to macOS Catalina 10.15, or installing a subsequent 10.15.x update, the auto_master file will be overwritten. You may need to re-append any custom lines to auto_master after the upgrade to restore your previously-working automounts.

Diagnose your automounts

Post-upgrade (or update), check the automount master mapping file (/etc/auto_master) to verify that your previously-working lines were removed.

sudo nano /etc/auto_master

The default contents probably look something like this:

#
# Automounter master map
#
+auto_master            # Use directory service
#/net                   -hosts          -nobrowse,hidefromfinder,nosuid
/home                   auto_home       -nobrowse,hidefromfinder
/Network/Servers        -fstab
/-                      -static

Fix your broken automounts

Optionally, save a backup copy of this original for future reference (and for quick restoration after each Catalina minor upgrade):

cd /etc
sudo cp auto_master auto_master.original

Now, append any lines you previously used to reference your other auto_* files. In my case, I have only one file named auto_nas.

/-                      auto_nas

Save your changes and then run the automount command to flush the cache and reload /etc/auto_master:

sudo automount -vc

Look for confirmation that each share mounted successfully:

automount: /System/Volumes/Data/Nas/media mounted
automount: /System/Volumes/Data/Nas/storage mounted
automount: /System/Volumes/Data/Nas/archives mounted

If you happen to be mounting any shares into the /System/Volumes/Data/ directory, make sure you have updated your mountpoints to match Catalina’s new directory structure. Otherwise, automount will complain about the mount point not existing.

UPDATE (2020-11-17)

Upgrading to Big Sur 11.x?

Good news! The upgrade process preserves your custom changes to the automount master mapping file (auto_master).