Well this issue has been on the back burner for a while so I've had another go at it.
I went back to basics and started again with the 120 Gbyte SSD and the USB3 caddy. I have installed Window 7 alongside 10 on my Laptop since my last attempt. I booted into 7, and used EaseUS Partition Master to format the drive to FAT32 file system with an MBR partition. As some of the replies suggested it seems it was the GPT partition issue which must have been causing the problem as I've just plugged it in, and set it to copy ten files to the SDD. So far so good. I couldn't seem to get it to create an MBR partition with Win 10, but MBR seems to be the default in Win 7.
Thanks for all the suggestions and I hope my post might help others, who might be faced with the same problem.