What happens when you have FastStartup on is that when you shut down, the current state of the PC is written to the Hyberfil.sys file, its like a shorter version of hibernate, In other words the HDD which contains the Hyberfil.sys file has to be up and running. Sleep mode may have something to do with it as you may get a conflict.
I am not a fan of FastStartup and keep it turned off as it has caused problems in the past when I have swapped drives. The very small difference it makes in the startup time is not worth the hassle. Unfortunately MS likes to turn it back on again with updates.