Everyone else in the world may know this, but I've only just discovered it! It may help someone.
I recently added an external USB hard drive to my system, to use for backing up. A couple of days ago I tried to overcome an unrelated problem by using System Restore - and it failed. I tried restoring to several different restore points - they all failed.
Turning this over in my mind, I realised that SR was monitoring both my internal HDD and my external USB HDD - and when I tried restoring to an earlier time, the USB drive was switched off. This must have been what screwed it up.
So I turned off SR on my backup drive (thinking about it, you wouldn't want that drive restored to an earlier time anyway, would you?). I then reset SR to start from scratch on my internal system drive, and created a new restore point. And it worked. SR now monitors only my system drive, and seems to restore when needed with no problems.
because I doubt that "Everyone else in the world" does know that.
On a similar theme - I use a USB Flash drive for archiving my Outlook email, it's handy to be able to slip the archive into my pocket when I'm travelling. Exactly the same thing applies - if the drive isn't attached to the computer when I launch Outlook there are immediate problems - Outlook always wants to load its archives when it launches, and if these are on an unplugged drive it makes a huge fuss, and pops up an error message. No amount of clicking will lose the message, and the only thing to do is find the drive and plug it in.
This may explain why I could not restore a few days ago. I have also recently installed a USB external drive for backup with Acronis. But,there always is a but,when I looked at System Restore Tab it just says (C:) monitoring in status,no other drive is listed.