diff --git a/storages/backends/s3boto3.py b/storages/backends/s3boto3.py
index 9caae4d0a..c5f721995 100644
--- a/storages/backends/s3boto3.py
+++ b/storages/backends/s3boto3.py
@@ -60,6 +60,8 @@ class S3Boto3StorageFile(File):
     buffer_size = setting('AWS_S3_FILE_BUFFER_SIZE', 5242880)
 
     def __init__(self, name, mode, storage, buffer_size=None):
+        if 'r' in mode and 'w' in mode:
+            raise ValueError("Can't combine 'r' and 'w' in mode.")
         self._storage = storage
         self.name = name[len(self._storage.location):].lstrip('/')
         self._mode = mode
@@ -91,7 +93,7 @@ def _get_file(self):
             )
             if 'r' in self._mode:
                 self._is_dirty = False
-                self._file.write(self.obj.get()['Body'].read())
+                self.obj.download_fileobj(self._file)
                 self._file.seek(0)
             if self._storage.gzip and self.obj.content_encoding == 'gzip':
                 self._file = GzipFile(mode=self._mode, fileobj=self._file, mtime=0.0)
@@ -141,7 +143,9 @@ def _flush_write_buffer(self):
             self._write_counter += 1
             self.file.seek(0)
             part = self._multipart.Part(self._write_counter)
-            part.upload(Body=self.file.read())
+            part.upload(Body=self.file)
+            self.file.seek(0)
+            self.file.truncate()
 
     def close(self):
         if self._is_dirty: