Its because the hard disk needs time to spin up before it can be used.
Bit like the tv valves that needed warming up before it kicked into life back in the 50's then ey? 
Mmmmmmm, that's progress in'it? 
But yer valve TV couldn't reproduce over 1,00 line sof picture or record two programmes simultaneously could it eh? EH?
WTF's a sof when it's at 'ome like?

I was watching some old vids...remember them

...whilst locked in me room in disgrace last night, and I must say the quality of them, even though some are over 20 yrs old, is still more than aadequate.

Who needs all this fancy pants gubbins ey?
I don't.

I'm easily pleased and happy about it thank you very much.