diff --git a/examples/companion_radio/DataStore.cpp b/examples/companion_radio/DataStore.cpp
index 06c56a7a44..60f719b9c7 100644
--- a/examples/companion_radio/DataStore.cpp
+++ b/examples/companion_radio/DataStore.cpp
@@ -80,7 +80,7 @@ void DataStore::begin() {
 
 #if defined(NRF52_PLATFORM) || defined(STM32_PLATFORM)
 int _countLfsBlock(void *p, lfs_block_t block){
-      if (block > _ContactsChannelsTotalBlocks) {
+      if (block >= _ContactsChannelsTotalBlocks) {
         MESH_DEBUG_PRINTLN("ERROR: Block %d exceeds filesystem bounds - CORRUPTION DETECTED!", block);
         return LFS_ERR_CORRUPT;  // return error to abort lfs_traverse() gracefully
     }
@@ -91,7 +91,9 @@ int _countLfsBlock(void *p, lfs_block_t block){
 
 lfs_ssize_t _getLfsUsedBlockCount(FILESYSTEM* fs) {
   lfs_size_t size = 0;
+  fs->_lockFS();
   int err = lfs_traverse(fs->_getFS(), _countLfsBlock, &size);
+  fs->_unlockFS();
   if (err) {
     MESH_DEBUG_PRINTLN("ERROR: lfs_traverse() error: %d", err);
     return 0;
diff --git a/examples/companion_radio/LockedLFS.h b/examples/companion_radio/LockedLFS.h
new file mode 100644
index 0000000000..56eb0a2090
--- /dev/null
+++ b/examples/companion_radio/LockedLFS.h
@@ -0,0 +1,52 @@
+#pragma once
+
+#include <CustomLFS.h>
+#include <InternalFileSystem.h>
+
+// InternalFS and a CustomLFS on internal flash use the same flash cache, which
+// has no lock of its own.  InternalFS uses it under its own lock, so take that
+// lock around all operations here too.
+class LockedLFS : public CustomLFS {
+  lfs_config orig;
+
+  static const lfs_config& base(const lfs_config* c) {
+    return static_cast<LockedLFS*>(static_cast<CustomLFS*>(c->context))->orig;
+  }
+
+  static int read(const lfs_config* c, lfs_block_t b, lfs_off_t o, void* buf, lfs_size_t n) {
+    InternalFS._lockFS();
+    int r = base(c).read(c, b, o, buf, n);
+    InternalFS._unlockFS();
+    return r;
+  }
+
+  static int prog(const lfs_config* c, lfs_block_t b, lfs_off_t o, const void* buf, lfs_size_t n) {
+    InternalFS._lockFS();
+    int r = base(c).prog(c, b, o, buf, n);
+    InternalFS._unlockFS();
+    return r;
+  }
+
+  static int erase(const lfs_config* c, lfs_block_t b) {
+    InternalFS._lockFS();
+    int r = base(c).erase(c, b);
+    InternalFS._unlockFS();
+    return r;
+  }
+
+  static int sync(const lfs_config* c) {
+    InternalFS._lockFS();
+    int r = base(c).sync(c);
+    InternalFS._unlockFS();
+    return r;
+  }
+
+public:
+  LockedLFS(uint32_t addr, uint32_t size, uint32_t block)
+    : CustomLFS(addr, size, block), orig(_lfs_config) {
+    _lfs_config.read = read;
+    _lfs_config.prog = prog;
+    _lfs_config.erase = erase;
+    _lfs_config.sync = sync;
+  }
+};
diff --git a/examples/companion_radio/main.cpp b/examples/companion_radio/main.cpp
index 89f0e6cb9f..375d9d7708 100644
--- a/examples/companion_radio/main.cpp
+++ b/examples/companion_radio/main.cpp
@@ -72,8 +72,8 @@ MultiSerialInterface interface_manager;
     DataStore store(InternalFS, QSPIFlash, rtc_clock);
   #else
     #if defined(EXTRAFS)
-      #include <CustomLFS.h>
-      CustomLFS ExtraFS(0xD4000, 0x19000, 128);
+      #include "LockedLFS.h"
+      LockedLFS ExtraFS(0xD4000, 0x19000, 128);
       DataStore store(InternalFS, ExtraFS, rtc_clock);
     #else
       DataStore store(InternalFS, rtc_clock);