I've been digging a little deeper into this and found that irstrtsv.exe is a service with the description "Flushes dirty pages on entry to standby" (whatever that means..) that can be disabled. So I disabled the service, which made my hard disk sleep again as it should. Even the sleep action went OK, but now the wake-up action took forever, it took maybe a minute with a lot of disk activity before my login-screen appeared.
irstrtsv.exe must do something with the amount of data written to the hibernation partition to make wake-up from standby much faster.
Question remains: Why is it accessing my secondary hard disk?
Edit: I see that my theory above is confirmed here: What is Intel® Rapid Start Technology? | Intel® Developer Zone
