diff --git a/SplitBit Test Manual.md b/SplitBit Test Manual.md
index dbbf339..0ee7f1a 100644
--- a/SplitBit Test Manual.md	
+++ b/SplitBit Test Manual.md	
@@ -350,7 +350,7 @@ them. Four forms:
 | Form | Meaning |
 | --- | --- |
 | `name.img` | Scratch. Removed before the run, so the emulator makes a blank one |
-| `disks/name.img` | A fixture built by `makedisks.sh`, used as it stands |
+| `disks/name.img` | A fixture built by `makedisks.sh`, copied fresh for the run |
 | `...:ro` | Attached write protected, so a test can check the *device* bars writes |
 | `...@N` | Given a latency of N cycles, so a test can check the filesystem waits |
 
@@ -358,6 +358,15 @@ The `@N` form deserves a note. Every other test runs with the disk's answer avai
 before the next instruction, which is the one condition under which failing to wait looks
 exactly like working.
 
+**Both kinds of disk are fresh for every test**, and the fixture was not always. It used to
+be handed to each test where it lay, and twenty four tests name `disks/cosmos.img` while
+several of them write to one. `romBoot` was the test that found it: its recorded output
+described a directory that `selfBoot` had made earlier in the same run, so it passed in a
+full run and failed on its own. **That is the worst way round for a test to be wrong** - the
+form nobody runs is the one that tells the truth - and the shape of it is worth naming,
+because it is the same shape as a program that works only because of what ran before it.
+The whole suite was checked one test at a time afterwards, and that was the only one.
+
 **keys** names a file in `Tests/input` to be fed to the console as a *keyboard* rather than
 as standard input, and the difference between those is the whole reason the field exists.
 
diff --git a/Tests/expected/romBoot.out b/Tests/expected/romBoot.out
index bc027b8..bf05c04 100644
--- a/Tests/expected/romBoot.out
+++ b/Tests/expected/romBoot.out
@@ -9,7 +9,7 @@ deleted it
 and it is gone
 finished
 > mkdir Notes
-cannot make that: check the path, the name, and whether it is taken
+made
 > cd Notes
 /Notes> dir
 0 files
diff --git a/Tests/input/cosmosEditKeys.in b/Tests/input/cosmosEditKeys.in
index 8a8a00e..d223682 100644
--- a/Tests/input/cosmosEditKeys.in
+++ b/Tests/input/cosmosEditKeys.in
@@ -1,7 +1,7 @@
-eco ok„ƒƒh
-Xecho ok„†
-echo o„…k
+eco ok„ƒƒh
+Xecho ok„†
+echo o„…k
 echo okX
-echo k‚o
-Xecho aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa„†…!
+echo k‚o
+Xecho aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa„†…!
 exit
diff --git a/Tests/run.sh b/Tests/run.sh
index 493ce74..1afd53f 100755
--- a/Tests/run.sh
+++ b/Tests/run.sh
@@ -248,9 +248,9 @@ while IFS='|' read -r name src mode stdin limit disk keys; do
                 EMUARGS+=(--keyboard "$INPUT/$keys")
             fi
             [ "$limit" != "-" ] && EMUARGS+=(--cycles "$limit")
-            # A disk starts fresh for every run, so a test cannot pass because of what a
-            # previous one left lying on it. The emulator makes the image if it is
-            # missing, which is what removing it first arranges for.
+            # A disk starts fresh for every test, so a test cannot pass because of what a
+            # previous one left lying on it. How that is arranged differs between a scratch
+            # image and a built fixture, and both are below.
             # ---- More than one, separated by a plus ----
             #
             # The machine has four drives, so a test may name up to four images and they
@@ -277,12 +277,24 @@ while IFS='|' read -r name src mode stdin limit disk keys; do
                 case "$DISKFILE" in
                     *@*) DISKWAIT="${DISKFILE##*@}"; DISKFILE="${DISKFILE%@*}" ;;
                 esac
-                # A name with a directory in it is one of the images makedisks.sh built,
-                # and is used as it stands. A bare name is scratch: it is removed first so
-                # that nothing a test writes can be seen by the next one, and the emulator
-                # makes a blank image in its place.
+                # ---- A fresh disk for every test, whichever kind it is ----
+                #
+                # A bare name is scratch: it is removed, and the emulator makes a blank image
+                # in its place.
+                #
+                # A name with a directory in it is one of the images makedisks.sh built, and
+                # it is COPIED rather than used where it lies. Twenty four tests name
+                # disks/cosmos.img and several of them write to it, so a test used to be able
+                # to hand the next one a disk with its leavings on. That is not hypothetical:
+                # romBoot's recorded output described a directory that selfBoot had made, so
+                # it passed in a full run and failed on its own, which is the worst way round
+                # for a test to be wrong.
                 case "$DISKFILE" in
-                    */*) ;;
+                    */*)
+                        mkdir -p "$BUILD/.fixture"
+                        cp "$BUILD/$DISKFILE" "$BUILD/.fixture/${DISKFILE##*/}"
+                        DISKFILE=".fixture/${DISKFILE##*/}"
+                    ;;
                     *) rm -f "$BUILD/$DISKFILE" ;;
                 esac
                 EMUARGS+=(--disk "$BUILD/$DISKFILE")