diff --git a/ports/stm32/storage.c b/ports/stm32/storage.c
index 06dc8d6859..aecef8acc6 100644
--- a/ports/stm32/storage.c
+++ b/ports/stm32/storage.c
@@ -185,7 +185,7 @@ bool storage_write_block(const uint8_t *src, uint32_t block) {
     }
 }
 
-mp_uint_t storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_blocks) {
+int storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_blocks) {
     #if defined(MICROPY_HW_BDEV_READBLOCKS)
     if (FLASH_PART1_START_BLOCK <= block_num && block_num + num_blocks <= FLASH_PART1_START_BLOCK + MICROPY_HW_BDEV_IOCTL(BDEV_IOCTL_NUM_BLOCKS, 0)) {
         return MICROPY_HW_BDEV_READBLOCKS(dest, block_num - FLASH_PART1_START_BLOCK, num_blocks);
@@ -200,13 +200,13 @@ mp_uint_t storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_bl
 
     for (size_t i = 0; i < num_blocks; i++) {
         if (!storage_read_block(dest + i * FLASH_BLOCK_SIZE, block_num + i)) {
-            return 1; // error
+            return -MP_EIO; // error
         }
     }
     return 0; // success
 }
 
-mp_uint_t storage_write_blocks(const uint8_t *src, uint32_t block_num, uint32_t num_blocks) {
+int storage_write_blocks(const uint8_t *src, uint32_t block_num, uint32_t num_blocks) {
     #if defined(MICROPY_HW_BDEV_WRITEBLOCKS)
     if (FLASH_PART1_START_BLOCK <= block_num && block_num + num_blocks <= FLASH_PART1_START_BLOCK + MICROPY_HW_BDEV_IOCTL(BDEV_IOCTL_NUM_BLOCKS, 0)) {
         return MICROPY_HW_BDEV_WRITEBLOCKS(src, block_num - FLASH_PART1_START_BLOCK, num_blocks);
@@ -221,7 +221,7 @@ mp_uint_t storage_write_blocks(const uint8_t *src, uint32_t block_num, uint32_t
 
     for (size_t i = 0; i < num_blocks; i++) {
         if (!storage_write_block(src + i * FLASH_BLOCK_SIZE, block_num + i)) {
-            return 1; // error
+            return -MP_EIO; // error
         }
     }
     return 0; // success
diff --git a/ports/stm32/storage.h b/ports/stm32/storage.h
index 2766ac59b3..490fc4a09b 100644
--- a/ports/stm32/storage.h
+++ b/ports/stm32/storage.h
@@ -47,9 +47,9 @@ void storage_flush(void);
 bool storage_read_block(uint8_t *dest, uint32_t block);
 bool storage_write_block(const uint8_t *src, uint32_t block);
 
-// these return 0 on success, non-zero on error
-mp_uint_t storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_blocks);
-mp_uint_t storage_write_blocks(const uint8_t *src, uint32_t block_num, uint32_t num_blocks);
+// these return 0 on success, negative errno on error
+int storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_blocks);
+int storage_write_blocks(const uint8_t *src, uint32_t block_num, uint32_t num_blocks);
 
 int32_t flash_bdev_ioctl(uint32_t op, uint32_t arg);
 bool flash_bdev_readblock(uint8_t *dest, uint32_t block);