If you bought your storage media from a less than reliable source (i.e Ebay) you need to check that its has all it claims. There is a freebie program that you can download from this url:
click here program will confirm that actual storage capacity for you.
As to why Memtest did not boot directly from your USB, make sure that you watch the screen - it may first come up with the legend:
'Press any key to boot from ......' If you don't react in time, Windows will boot normally.