--- a/xpcom/io/nsLocalFileUnix.cpp
+++ b/xpcom/io/nsLocalFileUnix.cpp
@@ -812,46 +812,80 @@ nsLocalFile::CopyToNative(nsIFile *newPa
 
 #ifdef DEBUG_blizzard
         int32_t totalRead = 0;
         int32_t totalWritten = 0;
 #endif
         char buf[BUFSIZ];
         int32_t bytesRead;
         
+        // record PR_Write() error for better error message later.
+        nsresult saved_write_error = NS_OK;    
+        nsresult saved_read_error = NS_OK;
+
+        // DONE: Does PR_Read() return bytesRead < 0 for error?
+        // Yes., The errors from PR_Read are not so common and
+        // the value may not have correspondence in NS_ERROR_*, but
+        // we do catch it still, immediately after while() loop.
+        // We can differentiate errors pf PR_Read and PR_Write by
+        // looking at saved_write_error value. If PR_Write error occurs (and not
+        // PR_Read() error), save_write_error is not NS_OK.
+
         while ((bytesRead = PR_Read(oldFD, buf, BUFSIZ)) > 0) {
 #ifdef DEBUG_blizzard
             totalRead += bytesRead;
 #endif
 
             // PR_Write promises never to do a short write
             int32_t bytesWritten = PR_Write(newFD, buf, bytesRead);
             if (bytesWritten < 0) {
+                saved_write_error = NSRESULT_FOR_ERRNO();
                 bytesRead = -1;
                 break;
             }
             NS_ASSERTION(bytesWritten == bytesRead, "short PR_Write?");
 
 #ifdef DEBUG_blizzard
             totalWritten += bytesWritten;
 #endif
         }
 
+        // Record error if PR_Read() failed.
+        // Must be done before any other I/O which may reset errno.
+        if ( (bytesRead < 0) && (saved_write_error == NS_OK)) {
+            saved_read_error = NSRESULT_FOR_ERRNO();
+        }
+
 #ifdef DEBUG_blizzard
         printf("read %d bytes, wrote %d bytes\n",
                  totalRead, totalWritten);
 #endif
 
+        // TODO/FIXME: better find out how to propagate errors of
+        // close.  Errors of close can occur.  Read man page of
+        // close(2); At least, we should tell the user that
+        // filesystem/disk is hosed so that users can take remedial
+        // action.
+
         // close the files
         PR_Close(newFD);
         PR_Close(oldFD);
 
-        // check for read (or write) error after cleaning up
-        if (bytesRead < 0) 
-            return NS_ERROR_OUT_OF_MEMORY;
+        // Let us report the failure to write and read.
+        // check for write/read error after cleaning up
+        if (bytesRead < 0) {
+            if (saved_write_error != NS_OK)
+                return saved_write_error;
+            else if (saved_read_error != NS_OK)
+                return saved_read_error;
+#if DEBUG
+            else                // sanity check. Die and debug.
+                MOZ_ASSERT(0);
+#endif
+        }
     }
     return rv;
 }
 
 NS_IMETHODIMP
 nsLocalFile::CopyToFollowingLinksNative(nsIFile *newParent, const nsACString &newName)
 {
     return CopyToNative(newParent, newName);