SOLVED
Thanx,
To explain in length:-
I have been trying for ages to create an w8.1 media rescue USB of my 1Tbite main HD.
I finally created one & to make sure it worked (having failed to make a virtual disk) I tried to test it on my spare w10 500gb HD but, it did not install w8.1 saying "Not enough space".
As a few helpful people suggested, I deleted all 6 partitions & reformatted the whole 500gb as one partition &
installed W8 on it.
I created an w8.1 media rescue USB stick, plugged it in & set it as 1st boot in the Bios.
Then rebooted & the test worked & w8 was updated to w8.1.
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Asus P5K Premium WiFiiFi, Intel Core 2 Quad Pro 2.4GHz, OCZ Cpu Cooler, 2x2GB, DDR2 800 PC2-6400, GeForce 9800 GTX (PCI-E), a Seagate 500gb & a WD1Tbite, OCZ 600W Psu, Win8/8.1/10