diff -ru a/CPP/7zip/Archive/GptHandler.cpp b/CPP/7zip/Archive/GptHandler.cpp
--- a/CPP/7zip/Archive/GptHandler.cpp	2026-10-09 18:08:33.721749900 -0400
+++ b/CPP/7zip/Archive/GptHandler.cpp	2026-10-09 18:08:55.074039300 -0400
@@ -2,6 +2,7 @@
 
 #include "StdAfx.h"
 
+#include <stdlib.h>
 #include "../../../C/7zCrc.h"
 #include "../../../C/CpuArch.h"
 
@@ -348,6 +349,19 @@
 IMP_IInArchive_Props
 IMP_IInArchive_ArcProps
 
+// johnmcm.com: with several partitions, 7-Zip opens none of them by itself.
+// JOHNMCM_PARTITION=<index> names the one to open, so a page can list a
+// partition's files in place instead of copying the partition out first.
+static int Johnmcm_ChosenPartition(unsigned count)
+{
+  const char *s = getenv("JOHNMCM_PARTITION");
+  if (!s || !*s)
+    return -1;
+  char *end;
+  const unsigned long i = strtoul(s, &end, 10);
+  return (*end == 0 && i < count) ? (int)i : -1;
+}
+
 Z7_COM7F_IMF(CHandler::GetArchiveProperty(PROPID propID, PROPVARIANT *value))
 {
   COM_TRY_BEGIN
@@ -358,6 +372,12 @@
     {
       if (_items.Size() == 1)
         prop = (UInt32)0;
+      else
+      {
+        const int chosen = Johnmcm_ChosenPartition(_items.Size());
+        if (chosen >= 0)
+          prop = (UInt32)chosen;
+      }
       break;
     }
     case kpidPhySize: prop = _totalSize; break;
diff -ru a/CPP/7zip/Archive/MbrHandler.cpp b/CPP/7zip/Archive/MbrHandler.cpp
--- a/CPP/7zip/Archive/MbrHandler.cpp	2026-10-09 18:08:33.729514400 -0400
+++ b/CPP/7zip/Archive/MbrHandler.cpp	2026-10-09 18:08:55.075038500 -0400
@@ -2,6 +2,7 @@
 
 #include "StdAfx.h"
 
+#include <stdlib.h>
 // #define SHOW_DEBUG_INFO
 
 #ifdef SHOW_DEBUG_INFO
@@ -529,6 +530,19 @@
 IMP_IInArchive_ArcProps
 
 
+// johnmcm.com: with several partitions, 7-Zip opens none of them by itself.
+// JOHNMCM_PARTITION=<index> names the one to open, so a page can list a
+// partition's files in place instead of copying the partition out first.
+static int Johnmcm_ChosenPartition(unsigned count)
+{
+  const char *s = getenv("JOHNMCM_PARTITION");
+  if (!s || !*s)
+    return -1;
+  char *end;
+  const unsigned long i = strtoul(s, &end, 10);
+  return (*end == 0 && i < count) ? (int)i : -1;
+}
+
 Z7_COM7F_IMF(CHandler::GetArchiveProperty(PROPID propID, PROPVARIANT *value))
 {
   NCOM::CPropVariant prop;
@@ -547,6 +561,12 @@
           }
           mainIndex = (int)i;
         }
+      if (mainIndex < 0)
+      {
+        const int chosen = Johnmcm_ChosenPartition(_items.Size());
+        if (chosen >= 0 && _items[(unsigned)chosen].IsReal)
+          mainIndex = chosen;
+      }
       if (mainIndex >= 0)
         prop = (UInt32)(Int32)mainIndex;
       break;