Index: uspace/lib/ext4/src/superblock.c
===================================================================
--- uspace/lib/ext4/src/superblock.c	(revision 32254d6ae36aa180fa53338ca60684a9c87d7947)
+++ uspace/lib/ext4/src/superblock.c	(revision 9faba42df60b89ae7031bd39ca9d3fb433fa989a)
@@ -1215,13 +1215,31 @@
 	uint64_t first_block = EXT4_SUPERBLOCK_OFFSET / phys_block_size;
 
-	/* Compute number of block to write */
-	size_t block_count = EXT4_SUPERBLOCK_SIZE / phys_block_size;
-
-	/* Check alignment */
-	if (EXT4_SUPERBLOCK_SIZE % phys_block_size)
-		block_count++;
-
-	/* Write data */
-	return block_write_direct(service_id, first_block, block_count, sb);
+	if (phys_block_size <= EXT4_SUPERBLOCK_SIZE) {
+		/* Superblock is a multiple of physical block sizes */
+		size_t block_count = EXT4_SUPERBLOCK_SIZE / phys_block_size;
+		return block_write_direct(service_id, first_block, block_count, sb);
+	}
+
+	/*
+	 * Superblock fills only a part of the physical block,
+	 * but we can only overwrite an entire physical block.
+	 */
+	void *tmp_sb = malloc(phys_block_size);
+	if (tmp_sb == NULL)
+		return ENOMEM;
+
+	/* Preserve physical block data */
+	rc = block_read_direct(service_id, first_block, 1, tmp_sb);
+	if (rc == EOK) {
+		/* Write the superblock */
+		void *sb_pos = tmp_sb + EXT4_SUPERBLOCK_OFFSET % phys_block_size;
+		memcpy(sb_pos, sb, EXT4_SUPERBLOCK_SIZE);
+
+		/* Write physical block to device */
+		rc = block_write_direct(service_id, first_block, 1, tmp_sb);
+	}
+
+	free(tmp_sb);
+	return rc;
 }