diff --git a/src/blockdev.c b/src/blockdev.c
index 443efcd..43c990c 100644
--- a/src/blockdev.c
+++ b/src/blockdev.c
@@ -1,5 +1,7 @@
 
 #include <stdio.h>
+#include <errno.h>
+#include <string.h>
 
 #include "blockdev.h"
 
@@ -11,10 +13,19 @@
 {
 	block_device = fopen(name, "rwb");
 	if (!block_device) {
+		fprintf(stderr, "devopen: %s.\n", strerror(errno));
 		return 0;
 	}
-	fseek(block_device, 0, SEEK_END);
+	int seek_res = 0;
+	seek_res |= fseek(block_device, 0, SEEK_END);
 	device_size = ftell(block_device);
+	seek_res |= fseek(block_device, 0, SEEK_SET);
+	if (seek_res || device_size == -1L || device_size % 512) {
+		fprintf(stderr, "devopen: Bad file %s.\n", name);
+		devclose();
+		return 0;
+	}
+
 	return 1;
 }