Before using Akeeba CMS Update you should be aware of some known issues which are outside our control and have to do with the way Joomla! itself and its download servers work. The same issues affect Joomla! Update, shipped with Joomla! itself: they are not unique to Akeeba CMS Update.
The download progress bar jumps from 0% to 100%. This is something we cannot fix on our site. Joomla!'s download server currently doesn't report the size of the Joomla! update / installation packages and doesn't support HTTP Range which would permit chunked download of the update file and correct update of the download progres bar.
Sometimes the download of the update fails cold. This is the same as the previous issue. Since Joomla!'s download server doesn't allow us to perform chunked download the entire update package has to be downloaded in a single page load. If you are upgrading to a new minor version (e.g. Joomla! 3.2 to 3.3) or from more than one subminor version (e.g. 3.2.1 to 3.2.3) you have to download the entire Joomla! package, nearly 9Mb long. On lower end servers, mostly shared hosting ones, this may take too long and lead to a timeout error. Unfortunately we cannot do anything from our side.
The post-update step may timeout on some servers. Again this is something we cannot fix on our end. The post-update step is actually a .php file shipped with Joomla! itself. It tries to remove obsolete files and folders and perform database upgrade in a single page load. Due to the way it's written it is IMPOSSIBLE to break down these time consuming actions into smaller steps. As a result lower end servers, mostly shared hosting ones, may time out while executing the post-update step. In this case you can MOST LIKELY fix your site by going to Extensions, Extensions Manager, Database and click on the Fix button. Since this is part of core Joomla! we regret that we cannot provide a fix.
This version requires PHP 5.3 or 5.4. The rationale behind this is explained in our statement of mid-February 2013. It won't install on hosts running PHP 5.2 or earlier. Moreover, due to the necessary Joomla! API changes found only in Joomla! 2.5.6 or later, this version will not install on Joomla! 2.5.5 or earlier versions. In any case, it will tell you exactly why it cannot be installed (minimum PHP or Joomla! version not satisfied).
This version of Akeeba Backup can be installed on Joomla! 2.5.6 or any later 2.5.x release, as well as any 3.2.x release. It is not compatible with Joomla! 3.0. It might work on Joomla! 3.1 but it has not been tested.