as far as my experience, there is no read/write caching during spin up, regardless if you are trying to read or write, your app will wait (sometimes appearing as 'hung') until the nas responds with the requested data read/write. The same thing happens when you have spin down on your PC hardrives.
nv+ ~ 1gb ram ~ 4x WDC WD20EARS-00S8B1 ~ 5555 GB ~ power supply dead
ultra4 ~ 4 gb ram ~
pro business ~ 4gb ram ~ 2x Seagate ST4000DX000-1C5160 ~ 2x Seagate ST4000DM000-1F2168 ~ 10tb
pro 6 ~ 4gb ram ~ dual redundancy ~ 4x Hitachi HDS724040ALE640 ~ 2x Seagate ST4000DM000-1F2168 ~ 14tb
A/V streaming ---> Subsonic
---> HTC One ~ Note 2 ~ Nexus 7