Hi George - thanks for your quick response.
I downloaded the SanDisk SSD Dashboard and it showed that the SSD Cache was not operational. All attempts to re-initialize it failed, so it seems like a hardware failure. I eventually disabled the SSD cache in the Device Manager.
The laptop now operates with the normal disk only - so it takes a long time to boot and is very slow.
I will now replace the disk with an SSD - since the cache is now disabled, there should be no problem.