Index: src/wp-admin/includes/class-wp-upgrader.php
===================================================================
--- src/wp-admin/includes/class-wp-upgrader.php	(revision 25861)
+++ src/wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -1369,7 +1369,7 @@
 		// Copy update-core.php from the new version into place.
 		if ( !$wp_filesystem->copy($working_dir . '/wordpress/wp-admin/includes/update-core.php', $wp_dir . 'wp-admin/includes/update-core.php', true) ) {
 			$wp_filesystem->delete($working_dir, true);
-			return new WP_Error( 'copy_failed_for_update_core_file', $this->strings['copy_failed'] );
+			return new WP_Error( 'copy_failed_for_update_core_file', __( 'The update cannot be installed because we will be unable to copy certain files. This is usually due to inconsistent file permissions.' ), 'wp-admin/includes/update-core.php' );
 		}
 		$wp_filesystem->chmod($wp_dir . 'wp-admin/includes/update-core.php', FS_CHMOD_FILE);
 
Index: src/wp-admin/includes/file.php
===================================================================
--- src/wp-admin/includes/file.php	(revision 25859)
+++ src/wp-admin/includes/file.php	(working copy)
@@ -622,9 +622,11 @@
 	 * A disk that has zero free bytes has bigger problems.
 	 * Require we have enough space to unzip the file and copy its contents, with a 10% buffer.
 	 */
-	$available_space = @disk_free_space( WP_CONTENT_DIR );
-	if ( $available_space && ( $uncompressed_size * 2.1 ) > $available_space )
-		return new WP_Error( 'disk_full_unzip_file', __( 'Could not copy files. You may have run out of disk space.' ), compact( 'uncompressed_size', 'available_space' ) );
+	if ( defined( 'DOING_CRON' ) && DOING_CRON ) {
+		$available_space = @disk_free_space( WP_CONTENT_DIR );
+		if ( $available_space && ( $uncompressed_size * 2.1 ) > $available_space )
+			return new WP_Error( 'disk_full_unzip_file', __( 'Could not copy files. You may have run out of disk space.' ), compact( 'uncompressed_size', 'available_space' ) );
+	}
 
 	$needed_dirs = array_unique($needed_dirs);
 	foreach ( $needed_dirs as $dir ) {
@@ -722,9 +724,11 @@
 	 * A disk that has zero free bytes has bigger problems.
 	 * Require we have enough space to unzip the file and copy its contents, with a 10% buffer.
 	 */
-	$available_space = @disk_free_space( WP_CONTENT_DIR );
-	if ( $available_space && ( $uncompressed_size * 2.1 ) > $available_space )
-		return new WP_Error( 'disk_full_unzip_file', __( 'Could not copy files. You may have run out of disk space.' ), compact( 'uncompressed_size', 'available_space' ) );
+	if ( defined( 'DOING_CRON' ) && DOING_CRON ) {
+		$available_space = @disk_free_space( WP_CONTENT_DIR );
+		if ( $available_space && ( $uncompressed_size * 2.1 ) > $available_space )
+			return new WP_Error( 'disk_full_unzip_file', __( 'Could not copy files. You may have run out of disk space.' ), compact( 'uncompressed_size', 'available_space' ) );
+	}
 
 	$needed_dirs = array_unique($needed_dirs);
 	foreach ( $needed_dirs as $dir ) {
Index: src/wp-admin/includes/update-core.php
===================================================================
--- src/wp-admin/includes/update-core.php	(revision 25861)
+++ src/wp-admin/includes/update-core.php	(working copy)
@@ -661,7 +661,7 @@
 	$versions_file = trailingslashit( $wp_filesystem->wp_content_dir() ) . 'upgrade/version-current.php';
 	if ( ! $wp_filesystem->copy( $from . $distro . 'wp-includes/version.php', $versions_file ) ) {
 		 $wp_filesystem->delete( $from, true );
-		 return new WP_Error( 'copy_failed_for_version_file', __( 'Could not copy file.' ) );
+		 return new WP_Error( 'copy_failed_for_version_file', __( 'The update cannot be installed because we will be unable to copy certain files. This is usually due to inconsistent file permissions.' ), 'wp-includes/version.php' );
 	}
 
 	$wp_filesystem->chmod( $versions_file, FS_CHMOD_FILE );
@@ -729,7 +729,7 @@
 			$error_data = version_compare( $old_wp_version, '3.7-beta2', '>' ) ? array_keys( $files_not_writable ) : '';
 
 			if ( $files_not_writable )
-				return new WP_Error( 'files_not_writable', __( 'Could not copy file.' ), $error_data );
+				return new WP_Error( 'files_not_writable', __( 'The update cannot be installed because we will be unable to copy certain files. This is usually due to inconsistent file permissions.' ), implode( ', ', $error_data ) );
 		}
 	}