Hi Corazon, thanks for your answer.
I understand that in the great scheme of things, leaving the cache on the SSD will probably make not a jot of difference to wear levels, all things considered.
However...I'm a bit OCD when it comes to getting something to work, and this is now really bugging me, so just forgetting about it isn't an option.
What I really don't understand is why all the other caches and temp files that I have moved can write data to the ramdrive, but the thumbs cache either can't or won't. I have to presume that something in junction is causing a problem, but is this because the ramdrive is seen differently to a normal HDD/SSD. Does windows consider it a removable drive, for instance.
If anyone using a ramdrive could replicate what I'm trying to do and see if they can solve the problem, I would be very happy, or at least tell me why it's not possible. As I have said, for me this is a learning process as much as the usefulness of getting it to work.
For what it's worth, the .db files contained in this cache file actually get pretty big. After cleaning the cache I can, within a minute or so, get it up to around 100MB (yes that is meg!) simply by clicking into folders containing jpegs. Have a look at your own cache size, you may be surprised!