diff --git a/features/TESTS/filesystem/fopen/fopen.cpp b/features/TESTS/filesystem/fopen/fopen.cpp
index f19c1c0c47..a0c95b3ae4 100644
--- a/features/TESTS/filesystem/fopen/fopen.cpp
+++ b/features/TESTS/filesystem/fopen/fopen.cpp
@@ -1231,6 +1231,24 @@ control_t fsfat_fopen_test_21(const size_t call_count)
     return CaseNext;
 }
 
+/** @brief  test for operation of SDFileSystem::format()
+ *
+ * @return on success returns CaseNext to continue to next test case, otherwise will assert on errors.
+ */
+control_t fsfat_fopen_test_22(const size_t call_count)
+{
+
+    FSFAT_FENTRYLOG("%s:entered\n", __func__);
+    (void) call_count;
+    int32_t ret = -1;
+
+    /* the allocation_unit of 0 means chanFS will use the default for the card (varies according to capacity). */
+    ret = sd.format(0);
+    FSFAT_TEST_UTEST_MESSAGE(fsfat_fopen_utest_msg_g, FSFAT_UTEST_MSG_BUF_SIZE, "%s:Error: failed to format sdcard (ret=%d)\n", __func__, (int) ret);
+    TEST_ASSERT_MESSAGE(ret == 0, fsfat_fopen_utest_msg_g);
+    return CaseNext;
+}
+
 
 
 
@@ -1319,6 +1337,7 @@ Case cases[] = {
 #ifdef FOPEN_NOT_IMPLEMENTED
         Case("FSFAT_FOPEN_TEST_21: todo.", FSFAT_FOPEN_TEST_21),
 #endif /* FOPEN_NOT_IMPLEMENTED */
+        Case("FSFAT_FOPEN_TEST_22: format() test.", FSFAT_FOPEN_TEST_22),
 
 };
 
diff --git a/features/filesystem/fat/FATFileSystem.cpp b/features/filesystem/fat/FATFileSystem.cpp
index 2e4e0ef9ff..21a6fb6b3b 100644
--- a/features/filesystem/fat/FATFileSystem.cpp
+++ b/features/filesystem/fat/FATFileSystem.cpp
@@ -323,6 +323,23 @@ int FATFileSystem::rename(const char *oldname, const char *newname) {
     return 0;
 }
 
+<<<<<<< HEAD
+=======
+/* See http://elm-chan.org/fsw/ff/en/mkfs.html for details of f_mkfs() and
+ * associated arguments. */
+int FATFileSystem::format(int allocation_unit) {
+    lock();
+    FRESULT res = f_mkfs(_fsid, 0, allocation_unit); // Logical drive number, Partitioning rule, Allocation unit size (bytes per cluster)
+    if (res) {
+        debug_if(FFS_DBG, "f_mkfs() failed: %d\n", res);
+        unlock();
+        return -1;
+    }
+    unlock();
+    return 0;
+}
+
+>>>>>>> b86fe65... STORAGE: change FATFileSystem::format() to include allocation_unit argument, to facilitate fixing of inconsistent file systems.
 DirHandle *FATFileSystem::opendir(const char *name) {
     lock();
     FATFS_DIR dir;
diff --git a/features/filesystem/fat/FATFileSystem.h b/features/filesystem/fat/FATFileSystem.h
index a0996a30f4..c66a4e66af 100644
--- a/features/filesystem/fat/FATFileSystem.h
+++ b/features/filesystem/fat/FATFileSystem.h
@@ -74,6 +74,18 @@ public:
      */
     virtual int rename(const char *oldname, const char *newname);
     
+    /**
+     * Formats a logical drive, FDISK partitioning rule.
+     *
+     * @param allocation_unit
+     *   This is the number of bytes per cluster size. The valid value is N
+     *   times the sector size. N is a power of 2 from 1 to 128 for FAT
+     *   volume and upto 16MiB for exFAT volume. If zero is given,
+     *   the default allocation unit size is selected depending on the volume
+     *   size.
+     */
+    virtual int format(int allocation_unit = 0);
+    
     /**
      * Opens a directory on the filesystem
      */