@RichieTheK & MontyJ
I have had no issues whatsoever with Calibre 64 with Windows 8 but that with spinning drives. That includes conversions, use of a number of plugins, tweaks, etc. The installation history has been Calibre 32/Windows 7>Upgrade to Windows 8 over Windows 7>Upgrade to Calibre 64 over Calibre 32.
You both have SSD's, so a common matter there and perhaps MontyJ's "adapter error" is a further clue.
I wonder (I may have missed seeing it) if MontyJ has ever had Windows 8 and Calibre 64 running fault free on his non-SSD drive but the rest of his environment unchanged?