If using MAC with Parallels (or similar Virtual Machine software) you may need to allow Windows to have more disc space. This is done within Parallels. If you have issues with Parallels you will need to seek advice for that software from the vendor, this is not covered by the Qest/Asyra support team or warranty.
One solution may be to locate the Backup folder on your hard drive. It is located in the main Qest4 or AsyraPro folder. Do this in the windows environment not mac.
Then select the old backup files you wish to delete.
You may also need to look at adjusting the settings within your parallels installation to increase the available memory for Windows.