I'm very puzzled by this. I was told by Apple on the phone that a back up to Time Machine would only happen when Im back hoe i.e. when my mac and time machine are on the same network ????
That sounds like they were thinking of the hardware item "Time Capsule" ( http://store.apple.com/us/product/ME182/airport-time-capsule-3tb) when you were talking about "Time Machine," a software feature. Apple's too-similar names confuse even their employees!
I use this external drive with my MacBook Pro for travel:
http://eshop.macsales.com/shop/firewire/EliteALmini/eSATA_FW800_FW400_USB
For scale, note the Apple Magic Mouse next to it. Mine weighs 14.5 ounces in spite of its stylish and sturdy aluminum case.
My external for the MBP has FW800 ports and USB 2. It is bus-powered via USB but I also have the optional external power brick (approx US$10) so the drive is more reliable when it is at home. I have experienced major issues with most of the "name-brand" externals that go on sale at the office superstore every weekend, but no issues whatsoever with the OWC products. We have an OWC external drive on each of our three most-used Intel Macs.
If you go for a "name-brand" drive, do not install the software that comes with it; it can slow your computer. Time Machine works just fine without any extra clutter on the hard drive. Also, connect the drive to your Mac, open Disk Utility, and reformat the drive to Mac HFS Extended. Most of the big-name drives come formatted for Windows. A Mac will read/write to them but the drive will be more reliable with Mac formatting.