Hi Stefan, The officially supported LSI-based controllers with the PE840 were the SAS 5iR and PERC 5/i. Neither of these supported spin-down, which was not available until the H700/H800 in 2009. If you have a different LSI-based controller (not necessarily a Dell-branded one) let me know which model and I might be able to assist finding documentation for the procedure to configure individual physical disk spin-down.
As a server, I've got a Power Edge 840 with a LSI SAS Raid Controller on a SAS backplane with 3 Seagate Cheetah 15K.5. There's debian running on a Raid 5.  The HDs don't spin down and even with megacli, I wasn't able to convince them to do so. Idling the server consumes 110W[NAME],6ghz, which is high. Any ideas?
Spin down HDs at PowerEdge LSI Raid