Offloading 100GB to a portable is never going to be quick.
Aside from the initial 100GB will the following backups be a completely fresh 100GB ie all new data, or just files that have been added or changed?
Maybe have a read of this first to get a feel.
http://www.acronis.com/en-gb/resource/s ... ckups.htmlIf it's a full clone you need each and every time, and if the laptop has easy access to the drive, you might consider taking the drive out and cloning it in a PC via a native sata dock.
With care though, this forum is littered with people asking for help having mixed up source and destination drives and realising all too late what they have done.
Also to consider, is backup over the network as noted by pclab.
gl.