diff --git a/README.md b/README.md
index c0b4880a..247babfd 100644
--- a/README.md
+++ b/README.md
@@ -642,8 +642,7 @@ The internal disk runs nearly full with the owner's own data, so builds go to th
 external drive, one folder per project: `/Volumes/Scratch/<project>/`. For example
 `dolphin-maps/` holds its checkout, `build/` (DerivedData, archives, result bundles:
 `BUILD_DIR` in its scripts), `tours/` (screenshots) and `scratch/` (logs, helpers).
-Simulator devices still live on the internal disk (CoreSimulator), so delete a tour's
-simulator after use. Never write a project's files into `~/src` loose.
+Never write a project's files into `~/src` loose.
 
 Temporary files follow the same rule. Anything a session creates with `mktemp` (a test
 run's DerivedData, a build's intermediates) lands in `TMPDIR`, which by default is on
@@ -654,6 +653,31 @@ its disk is mounted, for every zsh, non-interactive SSH sessions included; with
 or the drive unplugged, the system default stays. On this Mac:
 `ssh mac 'mkdir -p ~/.config/mac-setup && echo /Volumes/Scratch/tmp > ~/.config/mac-setup/tmpdir'`.
 
+Xcode's DerivedData works the same way: when `~/.config/mac-setup/deriveddata` names a folder
+on a mounted disk, `scripts/mac-setup.sh` sets Xcode's `IDECustomDerivedDataLocation` to it,
+which every `xcodebuild` without `-derivedDataPath` then uses. On this Mac:
+`ssh mac 'echo /Volumes/Scratch/DerivedData > ~/.config/mac-setup/deriveddata'`, then rerun
+`scripts/mac-setup.sh`. With the drive unplugged, Xcode builds fail rather than fill the
+internal disk.
+
+Simulator devices are on the drive too: `~/Library/Developer/CoreSimulator/Devices` is a
+symlink to `/Volumes/Scratch/CoreSimulator/Devices` (moved 2026-10-27, when they held 72 GB
+of the internal disk; free space went from 18 GB to 88 GB). CoreSimulator needs two things
+from the drive first, each a one-time step **(at the Mac)**, or it cannot create or read a
+device there ("device remained in Creating state", Cocoa error 513):
+- ownership on: `sudo diskutil enableOwnership /Volumes/Scratch`, then the drive's root back
+  to the user: `sudo chown "$(whoami)":staff /Volumes/Scratch`;
+- Full Disk Access for the simulator service: System Settings > Privacy & Security > Full Disk
+  Access, add `/Library/Developer/PrivateFrameworks/CoreSimulator.framework/Versions/A/XPCServices/com.apple.CoreSimulator.CoreSimulatorService.xpc`.
+
+Check it with a throwaway set before moving: `xcrun simctl --set /Volumes/Scratch/<dir>
+create probe "iPhone 17" <runtime>` must list and boot. Then: `rsync -aH --delete` the folder
+across while simulators run, `xcrun simctl shutdown all` and stop
+`com.apple.CoreSimulator.CoreSimulatorService`, sync again, require identical listings, swap
+in the symlink, and keep the old folder until `xcrun simctl list` shows the same device count.
+(`ditto` folds `._` files into extended attributes, so its copy cannot be compared file for
+file.) A tour's simulator is still deleted after use.
+
 ##### MacBook: Checks
 
 ```bash
diff --git a/scripts/mac/install.zsh b/scripts/mac/install.zsh
index f0e4c788..face7648 100644
--- a/scripts/mac/install.zsh
+++ b/scripts/mac/install.zsh
@@ -50,6 +50,22 @@ for t in swift xcodebuild xcrun; do
   [ "$got" = "$bin/$t" ] || { echo "mac-setup: a non-interactive zsh finds $t at $got, not $bin/$t" >&2; exit 1; }
 done
 
+# Xcode's DerivedData on the disk ~/.config/mac-setup/deriveddata names, when that file exists and the disk is
+# mounted; Xcode's default is ~/Library/Developer/Xcode/DerivedData, on the internal disk. It is a user default,
+# so it applies to Xcode and to every xcodebuild run without -derivedDataPath.
+dd_file=$HOME/.config/mac-setup/deriveddata
+if [ -r "$dd_file" ]; then
+  dd=$(<"$dd_file")
+  if [ -n "$dd" ] && [ -d "${dd:h}" ] && mkdir -p "$dd"; then
+    if [ "$(defaults read com.apple.dt.Xcode IDECustomDerivedDataLocation 2>/dev/null)" != "$dd" ]; then
+      defaults write com.apple.dt.Xcode IDECustomDerivedDataLocation "$dd"
+      echo "mac-setup: Xcode's DerivedData now goes to $dd"
+    fi
+  else
+    echo "mac-setup: WARNING: $dd_file names $dd, whose disk is not mounted; DerivedData left where it is" >&2
+  fi
+fi
+
 # The pinned Peekaboo app and CLI, and its LaunchAgent (the wrapper above runs its CLI).
 if [ "$update_peekaboo" = --update-peekaboo ]; then
   /bin/sh "$src/peekaboo-install.sh" --update-running