I have been testing a fully fanless system and had 2 160 gig segate sata 2 drives in Smart Drive 2002C HDD Silencer Enclosures. I stopped recording temperature when it reached 50c on idle. I intend to go back to using Zalman Heatpipe drive coolers . I have 4 other drives running with the Zalmans plus low fans and temps of 27c 24 c and 29c 26c ( lower temps nearer the fan) I have no idea how hot is hot for drives but would rather have a little fan noise and 30c than silence and 50c
Although it depends on what manufacturer and type of hard disk you have, I would say never go above 55*C. Desired is like 45* (or even lower of course), then you've got some play.
i have an antec 1200 case, the hard drives are right in front of a fan, a year ago i replaced 2 wd caviar (less than a year old) running loud at 48/50c with 2 samsung f2 satat II these have never topped 26c and are really quiet too.
Please not that this thread has been posted July 2nd, 2007 and hasn't received replies since February 2nd, 2008.