I’ve had problems with the new Seagate Momentus XT harddrive in my newest HTPC. The PC was locking up every 48 hours or so (which may have been related to standby), was occasionally jerky, and upon reboot would often refuse to boot with an error about a missing boot device, which could only be fixed with a hard power cycle. There is a firmware update for the drive to SD25 but this didn’t resolve the issue. However, after I changed the power management settings in Windows 7 to never spin-down the harddrive, the problems all went away. Success!
Update (14-Sept-2011). Seagate has released firmware SD28. There are reports that this fixes several issues and is a worthwhile update.