The disk has been verified, has shown no other problems and is up to date. Of the other volumes available in my system, only another (much smaller) with Snow Leopard shows as possible for the installation. The message for the disk I want to use is "This disk cannot be used to boot your computer". Computer is early 2009 MacPro with plenty of memory, disc to be used has 500 GB free. I have previously done a successfuil download of the installer, then a successful installation on another small volume, which is now erased, so I don't think there is a download problem. This installation was over Snow Leopard, which is what it now won't do on another volume.