Well the Laptop is not my main storage device but I do use it extensively and as Frank pointed out the K20D raw files plus all my other stuff just chews up space. And as long as I have the space I store most of my pics. It a great to be able to show them on a Laptop.
I have several USB external drives I use for backups of machine images, photos, music, etc. The Dell XPS is a huge power sucking beast to begin with so it is rarely on battery power - even at its peak you would be lucky to get a hour out of the battery (I have a workhorse 14" with a 9 Cell battery for portable power).
I have been running all week with both drives seems to work well. Having the extra HD space is nice and convenient. I always use a Laptop cooler on this machine as well as mentioned it is a power pig and normally runs hot.
I feel for ya Frank - I am on the computer all day every day (it's what I do) so I have multiple machines and normally switch them out every 2 - 4 years. I still remember my XT that would boot from a floppy for remote access to work. If I remember correctly it was about 12 minutes from power on to connection
Gooshin - I will probably replace the laptop before the price of large size storage SSD becomes a low cost mainstream ...
Heather best of luck on the new Desktop - generally much easier to upgrade but as you have now noticed with today's prices the upgrade should be cheap otherwise a replacement makes much more sense.
For me adding the HD was well worth it. YMMV