diff --git a/docs/API.md b/docs/API.md
index c110b568..ba8c6bcb 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -82,6 +82,11 @@ This feature is disabled by default. To enable it, set
   of flash sectors (default: 2);
 - `WOLFBOOT_DIAGNOSTICS_EXT`: setting this causes the region to live in
   external flash. Only meaningful when `EXT_FLASH` is enabled.
+- `WOLFBOOT_DIAGNOSTICS_RECORD_SIZE`: the on-flash slot size for each record and
+  the sector header, in bytes (default: 16). It must be a multiple of, and no
+  smaller than, the flash write granularity. Raise it on platforms whose flash
+  write word is wider than 16 bytes (for example 32 for the 256-bit words on
+  STM32H7).
 
 The region is managed as a circular store over its sectors. With two or more
 sectors, older records are retained until the log wraps all the way around, so
diff --git a/include/wolfboot/wolfboot.h b/include/wolfboot/wolfboot.h
index 5ce931f0..8f5c3ea2 100644
--- a/include/wolfboot/wolfboot.h
+++ b/include/wolfboot/wolfboot.h
@@ -630,8 +630,7 @@ int wolfBoot_get_partition_state(uint8_t part, uint8_t *st);
 #define WOLFBOOT_FAILURE_CAUSE_NOT_CONFIRMED 4 /* image never confirmed via
                                                 * wolfBoot_success() */
 
-/* Persisted failure record. Exactly 16 bytes so it maps to a single 128-bit
- * write-once flash word and can be appended without read-modify-write. */
+/* Persisted failure record. */
 struct wolfBoot_failure_record {
     uint32_t seq;        /* monotonic sequence number (higher = newer) */
     uint8_t  phase;      /* WOLFBOOT_FAILURE_PHASE_* */
diff --git a/options.mk b/options.mk
index 8b9b1457..849fc3c6 100644
--- a/options.mk
+++ b/options.mk
@@ -78,6 +78,9 @@ ifeq ($(WOLFBOOT_PERSIST_FAILURE_STATUS),1)
     ifneq ($(WOLFBOOT_DIAGNOSTICS_SECTORS),)
         CFLAGS+=-D"WOLFBOOT_DIAGNOSTICS_SECTORS=$(WOLFBOOT_DIAGNOSTICS_SECTORS)"
     endif
+    ifneq ($(WOLFBOOT_DIAGNOSTICS_RECORD_SIZE),)
+        CFLAGS+=-D"WOLFBOOT_DIAGNOSTICS_RECORD_SIZE=$(WOLFBOOT_DIAGNOSTICS_RECORD_SIZE)"
+    endif
     ifeq ($(WOLFBOOT_DIAGNOSTICS_EXT),1)
         CFLAGS+=-D"WOLFBOOT_DIAGNOSTICS_EXT"
     endif
diff --git a/src/libwolfboot.c b/src/libwolfboot.c
index 61a1de5b..95f91128 100644
--- a/src/libwolfboot.c
+++ b/src/libwolfboot.c
@@ -939,8 +939,12 @@ void RAMFUNCTION wolfBoot_success(void)
 #define DIAG_SECTOR_SIZE    WOLFBOOT_SECTOR_SIZE
 #define DIAG_SECTOR_ADDR(i) ((haladdr_t)(WOLFBOOT_DIAGNOSTICS_ADDRESS) + \
                              (haladdr_t)(i) * DIAG_SECTOR_SIZE)
-#define DIAG_HDR_SIZE       16U
-#define DIAG_RECORD_SIZE    16U
+
+#ifndef WOLFBOOT_DIAGNOSTICS_RECORD_SIZE
+#define WOLFBOOT_DIAGNOSTICS_RECORD_SIZE 16U
+#endif
+#define DIAG_HDR_SIZE       WOLFBOOT_DIAGNOSTICS_RECORD_SIZE
+#define DIAG_RECORD_SIZE    WOLFBOOT_DIAGNOSTICS_RECORD_SIZE
 #define DIAG_SLOTS_PER_SECTOR ((DIAG_SECTOR_SIZE - DIAG_HDR_SIZE) / DIAG_RECORD_SIZE)
 
 struct wolfBoot_diag_header {
@@ -950,11 +954,11 @@ struct wolfBoot_diag_header {
     uint32_t crc; /* CRC32 over the preceding 12 bytes */
 };
 
-/* Ensure both structures are exactly 128-bit */
 typedef char diag_record_size_check[
-    (sizeof(struct wolfBoot_failure_record) == DIAG_RECORD_SIZE) ? 1 : -1];
+    (sizeof(struct wolfBoot_failure_record) <= DIAG_RECORD_SIZE) ? 1 : -1];
 typedef char diag_header_size_check[
-    (sizeof(struct wolfBoot_diag_header) == DIAG_HDR_SIZE) ? 1 : -1];
+    (sizeof(struct wolfBoot_diag_header) <= DIAG_HDR_SIZE) ? 1 : -1];
+typedef char diag_record_min_check[(DIAG_RECORD_SIZE >= 16U) ? 1 : -1];
 
 static uint32_t RAMFUNCTION diag_crc32(const void *data, uint32_t len)
 {
@@ -1098,12 +1102,15 @@ static int RAMFUNCTION diag_write(haladdr_t addr, const void *buf, uint32_t len)
 static int RAMFUNCTION diag_write_header(haladdr_t sector_addr, uint32_t generation)
 {
     struct wolfBoot_diag_header hdr;
+    uint8_t slot[DIAG_HDR_SIZE];
     XMEMSET(&hdr, 0, sizeof(hdr));
     hdr.magic = DIAG_HDR_MAGIC;
     hdr.generation = generation;
     hdr.format_version = DIAG_FORMAT_VERSION;
     hdr.crc = diag_crc32(&hdr, 12);
-    return diag_write(sector_addr, &hdr, sizeof(hdr));
+    XMEMSET(slot, 0xFF, sizeof(slot));
+    XMEMCPY(slot, &hdr, sizeof(hdr));
+    return diag_write(sector_addr, slot, DIAG_HDR_SIZE);
 }
 
 /* Highest sequence number present across all sectors (0 if none). */
@@ -1128,6 +1135,7 @@ int RAMFUNCTION wolfBoot_record_failure(uint8_t phase, uint8_t cause,
     uint32_t gen[DIAG_N_SECTORS];
     int count[DIAG_N_SECTORS];
     struct wolfBoot_failure_record rec;
+    uint8_t slot[DIAG_RECORD_SIZE];
     uint32_t seq, active_gen;
     int active, active_count, ret;
 
@@ -1169,8 +1177,10 @@ int RAMFUNCTION wolfBoot_record_failure(uint8_t phase, uint8_t cause,
     rec.fw_version = fw_version;
     rec.crc = diag_crc32(&rec, 12);
 
+    XMEMSET(slot, 0xFF, sizeof(slot));
+    XMEMCPY(slot, &rec, sizeof(rec));
     ret = diag_write(DIAG_SECTOR_ADDR(active) + DIAG_HDR_SIZE +
-            (uint32_t)active_count * DIAG_RECORD_SIZE, &rec, sizeof(rec));
+            (uint32_t)active_count * DIAG_RECORD_SIZE, slot, DIAG_RECORD_SIZE);
 
     diag_lock();
     return ret;
diff --git a/tools/unit-tests/Makefile b/tools/unit-tests/Makefile
index 5fbd4ed3..e61e232b 100644
--- a/tools/unit-tests/Makefile
+++ b/tools/unit-tests/Makefile
@@ -65,6 +65,7 @@ TESTS:=unit-parser unit-fdt unit-extflash unit-string unit-spi-flash unit-aes128
 TESTS+=unit-tpm-check-rot-auth
 TESTS+=unit-tpm-api-names
 TESTS+=unit-diagnostics
+TESTS+=unit-diagnostics-256
 TESTS+=unit-fit-gzip unit-fit-nogzip
 TESTS+=unit-fit-fpga
 TESTS+=unit-mpusize
@@ -135,6 +136,7 @@ unit-fdt:CFLAGS+=-DWOLFBOOT_FDT
 unit-nvm:CFLAGS+=-DNVM_FLASH_WRITEONCE -DMOCK_PARTITIONS
 unit-nvm-flagshome:CFLAGS+=-DNVM_FLASH_WRITEONCE -DMOCK_PARTITIONS -DFLAGS_HOME
 unit-diagnostics:CFLAGS+=-DMOCK_PARTITIONS
+unit-diagnostics-256:CFLAGS+=-DMOCK_PARTITIONS -DWOLFBOOT_DIAGNOSTICS_RECORD_SIZE=32
 unit-enc-nvm:CFLAGS+=-DNVM_FLASH_WRITEONCE -DMOCK_PARTITIONS -DEXT_ENCRYPTED \
 	-DENCRYPT_WITH_CHACHA -DEXT_FLASH -DHAVE_CHACHA
 unit-enc-nvm:WOLFCRYPT_SRC+=$(WOLFBOOT_LIB_WOLFSSL)/wolfcrypt/src/chacha.c
@@ -456,6 +458,9 @@ unit-nvm-flagshome: ../../include/target.h unit-nvm.c
 unit-diagnostics: ../../include/target.h unit-diagnostics.c
 	gcc -o $@ unit-diagnostics.c $(CFLAGS) $(LDFLAGS)
 
+unit-diagnostics-256: ../../include/target.h unit-diagnostics.c
+	gcc -o $@ unit-diagnostics.c $(CFLAGS) $(LDFLAGS)
+
 unit-enc-nvm: ../../include/target.h unit-enc-nvm.c
 	gcc -o $@ $(WOLFCRYPT_SRC) unit-enc-nvm.c $(CFLAGS) $(WOLFCRYPT_CFLAGS) $(LDFLAGS)