external hdd setup questions

I've never owned a external hard drive before and am uncertain if what I have planned will work.

I have a MBP connected to a home network with a Airport Extreme base station. I want to get a HDD for Time Machine backups that I can USB attach to my Airport station for wireless backups. My questions:

1. do I need to worry about a HDD continuously running if its attached to my Airport base? Or will it only run when backing up?

2. Can I partition the HDD into 2 drives. One for Time Machine backups and one for Vault backups of my Aperture 2 library?

3. Can I get a 2.5 inch drive to save space and power it off of the Airport base? That will give me a drive with mobility if needed.

4. I am considering building my own, but how are OWC drives? Any experience with them?

Thanks for any advice.

1. The drive will run continuously but that should cause no harm or unnecessary wear and tear.

2. Yes. Partition the drive using Disk Utility. Remember, Time Machine will only work with a drive formatted to HFS+.

3. Probably not. 2.5" drives over ~250 - 320 GB are generally powered using an AC adapter rather than from USB power. An AC powered (from adapter) drive is more reliable anyway and will not cause problems with the USB ports.

4. I have never used a drive from OWC, however, the drives they sell are standard and can be purchased from just about anywhere. Shop around.

Regards.