diff --git a/Programs/CosmOS/Source/sbfs.asm b/Programs/CosmOS/Source/sbfs.asm
index d8b8d74..c65af02 100644
--- a/Programs/CosmOS/Source/sbfs.asm
+++ b/Programs/CosmOS/Source/sbfs.asm
@@ -1413,10 +1413,37 @@ sbfsBoundsDone:
 ; First fit, walking the directory, because with files laid down contiguously the
 ; directory already says which blocks are spoken for. There is no allocation table to
 ; consult and none to keep right.
+;
+; ---- Moving the candidate along without starting again ----
+;
+; This used to give up the moment it found something in the way: it moved the candidate past
+; that one entry and STARTED THE DIRECTORY AGAIN FROM THE FIRST BLOCK. With files laid down
+; one after another that is a restart per file, and every restart reads directory blocks off
+; the disk until it reaches the next one in the way - which is further in each time. A disk
+; of 183 files cost thousands of block reads to place one file, and assembling onto a disk
+; with the whole source tree on it took eleven minutes with nearly all of it spent here.
+;
+; The candidate moves along DURING the pass now, and the pass carries on from where it is.
+; Entries later in the directory are then tested against where the candidate has got to, so
+; on a disk that has been appended to - which is what a disk mostly is - one pass walks it
+; past everything and the next confirms there is nothing left in the way. Two passes rather
+; than one per file.
+;
+; IT IS STILL FIRST FIT. The candidate only ever moves past something that genuinely
+; overlaps it, and when it does there is nowhere below that could have held the run: the
+; entry in the way covers everything up to its end, and it begins before the candidate ends.
+; So nothing is skipped that first fit would have found - and because entries earlier in the
+; directory were tested against an earlier candidate, the pass repeats until one goes by with
+; the candidate standing still.
 sbfsAllocate:
   CALL sbfsFirstData
 
 sbfsAllocTry:
+  ; The candidate has not moved yet this time round.
+  SETD.0 SbfsAllocMoved
+  RSTA
+  STA.0
+
   SETD.0 SbfsCandEnd
   SETD.2 SbfsCandidate
   CALL sbfsSetWord
@@ -1491,12 +1518,26 @@ sbfsAllocEntry:
   CALL sbfsCompareWord
   BNC sbfsAllocClear
 
-  ; They do overlap, so try again from the far end of whatever is in the way.
-  POPD.2
+  ; They do overlap, so the candidate moves to the far end of what is in the way - and the
+  ; scan carries on from here rather than beginning again, so whatever comes next is measured
+  ; against where the candidate has got to.
+  ;
+  ; WHERE THIS ENTRY IS STAYS ON THE STACK THROUGHOUT. The work below wants DP2 for its own
+  ; purposes and the scan needs it back on the entry to step to the next one, which is what
+  ; sbfsAllocClear is for.
   SETD.0 SbfsCandidate
   SETD.2 SbfsEntryEnd
   CALL sbfsSetWord
-  BRI sbfsAllocTry
+  SETD.0 SbfsCandEnd
+  SETD.2 SbfsCandidate
+  CALL sbfsSetWord
+  SETD.0 SbfsCandEnd
+  SETD.2 SbfsWantBlocks
+  CALL sbfsAddWord
+  INIA 0x01
+  SETD.0 SbfsAllocMoved
+  STA.0
+  ; And on into sbfsAllocClear, which puts the entry back in DP2 and steps to the next one.
 
 sbfsAllocClear:
   POPD.2
@@ -1517,6 +1558,16 @@ sbfsAllocNext:
   STA.1
   BNA sbfsAllocBlock
 
+  ; ---- The pass is over ----
+  ;
+  ; Something was in the way, so start again from the far end of the furthest of them. That
+  ; is one jump for however many files the candidate ran into, rather than one jump each.
+  SETD.0 SbfsAllocMoved
+  LDA.0
+  BRA sbfsAllocRoom             ; It never moved, so nothing is in the way of where it is.
+  BRI sbfsAllocTry
+
+sbfsAllocRoom:
   ; Nothing was in the way, so this is where it goes.
   SETD.0 SbfsFileStart
   SETD.2 SbfsCandidate
@@ -3087,6 +3138,9 @@ SbfsCandidate:
   0x00 0x00
 SbfsCandEnd:
   0x00 0x00
+; Whether the candidate had to move at all during the pass just finished. See sbfsAllocate.
+SbfsAllocMoved:
+  0x00
 SbfsEntryStart:
   0x00 0x00
 SbfsEntryEnd:
diff --git a/Tests/makedisks.sh b/Tests/makedisks.sh
index 957e516..1924b35 100755
--- a/Tests/makedisks.sh
+++ b/Tests/makedisks.sh
@@ -393,7 +393,13 @@ tail -c +17 "$WORK/slotData.sbx" > "$WORK/slotData.raw"
 # ordinary file. Nothing here is a boot-specific format: /System/cosmos.bin is the same
 # SPBT image the emulator has always been handed directly, which is what makes a program
 # that wants no operating system startable the same way.
-"$TOOL" format "$DISKS/selfboot.img" 512 4 32 >/dev/null
+# ---- Forty blocks a slot, the same as a disk that ships ----
+#
+# It was thirty-two, which was ten thousand bytes of headroom when stage two was four
+# thousand bytes and none at all when it reached 8,234. A fixture tighter than the thing it
+# stands in for is a fixture that fails on a change the real disk would have taken, and the
+# failure says "the boot slot is too small" rather than what actually grew.
+"$TOOL" format "$DISKS/selfboot.img" 512 4 40 >/dev/null
 "$ROOT/Assembler" -I "$ROOT/Programs/CosmOS/Source" \
     "$ROOT/Programs/Boot/stage2.asm" -o "$WORK/stage2.sbx" >/dev/null
 tail -c +17 "$WORK/stage2.sbx" > "$WORK/stage2.raw"
@@ -413,7 +419,7 @@ tail -c +17 "$WORK/stage2.sbx" > "$WORK/stage2.raw"
 
 # And one with no system on it, so that a second stage which cannot find what to start
 # says so rather than jumping somewhere.
-"$TOOL" format "$DISKS/nosystem.img" 512 4 32 >/dev/null
+"$TOOL" format "$DISKS/nosystem.img" 512 4 40 >/dev/null
 "$TOOL" boot "$DISKS/nosystem.img" "$WORK/stage2.raw" 0 >/dev/null
 
 # ---- Configuration choosing what starts ----