diff --git a/debian/changelog b/debian/changelog
index f08dc61..bfb4b83 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -15,6 +15,7 @@ flexnbd (0.2.0) UNRELEASED; urgency=medium
   * Proxy passes all NBD protocol errors through to the client instead of
     disconnecting and retrying (#36, !40)
   * Fix struct types in readwrite.c (#35, !41)
+  * Ignore ends of discs that stray outside of 512-byte sector sizes (!42). 
 
  -- James Carter <james.carter@bytemark.co.uk>  Thu, 11 Jan 2018 10:05:35 +0000
 
diff --git a/src/common/ioutil.c b/src/common/ioutil.c
index 4c1a4bd..dfa47df 100644
--- a/src/common/ioutil.c
+++ b/src/common/ioutil.c
@@ -97,6 +97,15 @@ int open_and_mmap(const char* filename, int* out_fd, uint64_t *out_size, void **
 		warn("lseek64() failed");
 		return size;
 	}
+
+	/* If discs are not in multiples of 512, then odd things happen,
+	 * resulting in reads/writes past the ends of files.
+	 */
+	if ( size != (size & (~0x1ff))) {
+		warn("file does not fit into 512-byte sectors; the end of the file will be ignored.");
+		size &= ~0x1ff;
+	}
+
 	if (out_size) {
 		*out_size = size;
 	}
diff --git a/src/server/serve.c b/src/server/serve.c
index d88e9f4..3a86c3a 100644
--- a/src/server/serve.c
+++ b/src/server/serve.c
@@ -710,6 +710,15 @@ void serve_init_allocation_map(struct server* params)
 
 	FATAL_IF_NEGATIVE(fd, "Couldn't open %s", params->filename );
 	size = lseek64( fd, 0, SEEK_END );
+
+	/* If discs are not in multiples of 512, then odd things happen,
+	 * resulting in reads/writes past the ends of files.
+	 */ 
+	if ( size != (size & ~0x1ff)) {
+		warn("file does not fit into 512-byte sectors; the end of the file will be ignored.");
+		size &= ~0x1ff;
+	}
+
 	params->size = size;
 	FATAL_IF_NEGATIVE( size, "Couldn't find size of %s",
 			   params->filename );
diff --git a/tests/acceptance/test_serve_mode.rb b/tests/acceptance/test_serve_mode.rb
index 4f9a77e..2299584 100644
--- a/tests/acceptance/test_serve_mode.rb
+++ b/tests/acceptance/test_serve_mode.rb
@@ -195,4 +195,18 @@ class TestServeMode < Test::Unit::TestCase
     assert_equal 6, op.first[3], 'msync called with incorrect flags'
   end
 
+  def test_odd_size_discs_are_truncated_to_nearest_512
+    # This should get rounded down to 1024
+    @env.blocksize = 1024 + 511
+    @env.writefile1('0')
+    @env.serve1
+    client = FlexNBD::FakeSource.new(@env.ip, @env.port1, 'Connecting to server failed')
+    begin
+      result = client.read_hello
+      assert_equal 'NBDMAGIC', result[:passwd]
+      assert_equal 0x00420281861253, result[:magic]
+      assert_equal 1024, result[:size]
+      client.close
+    end
+  end
 end