Login error…

My client has just contacted me to tell me they're getting this error message:

The following error occurred: ALTER TABLE perch2_categories CHANGE catDisplayPath catDisplayPath CHAR(255) NOT NULL DEFAULT '' SQLSTATE[HY000]: General error: 3 Error writing file './gustoresearch/#sql-db1_21cf.frm' (Errcode: 28)

I updated the site about a month ago and here are the diagnostics:

Perch: 2.8.29, PHP: 5.6.10, MySQL: mysqlnd 5.0.11-dev - 20120503 - $Id: 3c688b6bbc30d36af3ac34fdd4b7b5b787fe5555 $, with PDO
Server OS: Darwin, apache2handler
Installed apps: content (2.8.29), assets (2.8.29), categories (2.8.29), perch_blog (5.0), perch_forms (1.8.3), perch_members (1.5), perch_backup (1.2)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_forms', 'perch_members', 'perch_blog', ); ?>
PERCH_LOGINPATH: /cms
PERCH_PATH: /Users/TemperTemper/Sites/Gusto/gustoresearch.com/cms
PERCH_CORE: /Users/TemperTemper/Sites/Gusto/gustoresearch.com/cms/core
PERCH_RESFILEPATH: /Users/TemperTemper/Sites/Gusto/gustoresearch.com/cms/resources
Image manipulation: GD
PHP limits: Max upload 32M, Max POST 32M, Memory: 128M, Total max file upload: 32M
F1: 2edba60ed1f613d6dd804feb202456a2
Resource folder writeable: Yes
HTTP_HOST: gustoresearch.local
DOCUMENT_ROOT: /Users/TemperTemper/Sites/Gusto/gustoresearch.com
REQUEST_URI: /cms/core/settings/diagnostics/
SCRIPT_NAME: /cms/core/settings/diagnostics/index.php

Is it ok to 'accept these errors and continue'? Or has something sinister happened…?

Thanks,

Martin.