after cogitating for some time I still can't get my head around how two Linux Mint installations, each on it's own hard disk, not in the system at the same time, can have become corrupted in the same way.
Perhaps the disks are not corrupted. I have re-installed Linux Mint to one of them and it is no different.
Starting from this position I have tried to isolate what else in the system could have become US. What I don't know is whether anything a Linux Mint OS does or it's application programmes do can have caused a configuration change to other parts of the system, ie does the graphics card retain set up info during power off situations? Does the BIOS get changed while the machine runs?
So I have swapped graphics cards, no change. I have tried removing each memory module and running up the system on one at a time, no change.
As the machine is as good as useless I decided to go the whole hog and replaced the mother board and CPU with some faster parts I bought a while ago but never got around to fitting. So, here I am, booted up from the Linux Live CD, everything doing what it is supposed to do except that it won't fully boot up from the same S/W installed on the hard disk drive(s) from the same Live CD. It just fails to do it at 3.3GHz instead of 2.4GHz.
It still boots into WinXP from another hard disk and runs my application programmes.
To re-cap. From the Linux Mint installed drives the system boots normally up to the entry of user name and password, which it accepts, and then it progresses nearly to the desktop display, stalling with the mouse pointer working on an otherwise totally black screen. The system has not crashed. When the power button is pressed momentarily a small amount of text appears at the top of the screen prompting for login and showing the volume name on the hard disk and then it writes shutting down info and then the power supply switches off. The behaviour is identical for each hard disk, one of which has much application software and data and one which has just a new Linux install on. The hard disks are all in trays and can be put in or removed easily.
More confused than ever......
Rans6.......