No matter how hard you try, you wont be able to eliminate all activities that are preventing HDD from spinning down. There are cron / anacron jobs, auto-updates, heck, even screensaver can trigger HDD. Ive tried this in my netbook, to conserve energy, and its simply not possible without some serious hack and fine-tuning.

So yes, SSD or USB are great solutions for silent operation. My pros and cons:

SSD

Pros:

• Speed - Its the fastest non volatile storage you can get, period


• Size - It starts from 32~64GB and all the way to 256GB (or even more)


• Compatibility - Any motherboard with Sata will handle them

Cons:

• Price - They are still very expensive compared to HDD. Prolly overkill to an htpc


• Install - You have to open you machine, screw it, plug wires, etc etc

USB Stick

Pros:

• Price - Very affordable, and you prolly have a few 4~16GB lying around already


• Plug and Play - Stick it and boot. No wires, no opening your case


• Portable - Take it to any PC and you have your portable htpc server ;)

Cons:

• Size: Usually top USB sticks are 32~64GB. Not that you need more for an htpc


• Blinking led: Try to find one without led, or it will be annoying. Sandisk's Cruzer Blade may be an option


• Ancient BIOS may not boot from USB. Well, they may not have Sata either, so not really a con compared to SSD


• Speed - Even the fastest USB2.0 is no match for an SSD

May I add a 3rd contender?

SD Flash Card

Pros

• Cheapest of all.


• Pretty decent speed of you get a good class card


• Put that never-used card reader to some good use ;)


• Don't use one of your precious USB slots

Cons

• Need a card reader (duh)


• Not good if you actually use your card reader in your htpc


• Smallest size of the bunch

Hope that helps!