Okay 47 views and nothing
What I'm wondering is the platter density very low therefore the drive is slow? I'm finding it difficult to get any specs on this drive as its from an Xbox 360
Looks like that drive is from the Samsung M40S Series:
Based on 40GB platters. So the 20GB version you have is a 1 head/1 platter.
I don't know why you're concerned if it's slow. It's for a HTPC. You shouldn't be loading and running a ton of programs on it. It should be just for the OS and your media player. The only thing that might make it slow is those superfetech/indexing services that'll trash a HD, so it's best to turn those off. But since you're planning on using Ubuntu and not Windows, it shouldn't be any problems at all.
Anyways, I plan to use a left over 80GB 5400rpm Fujitsu laptop HD I have from my laptop replacement as the OS drive for the HTPC I'm planning to build. I have a 1TB and a 1.5TB for media storage. If SSD prices come down some more, might replace the OS drive with one. I care about silence, don't really care about speed at all.