To spell it out then for some, I'm a photographer and it would be very useful to always have my archive image library on my laptop - so my images are always there wherever I go (I go everywhere with my laptop!) for whenever a client rings me up asking for images (without having to remember to carry and regularly update an external disk)! It's about 300GB in size and growing about about 50GB a year.
Maybe an alternative would be to upload copies of the files that would be on an external drive at home to "the cloud?" You would have access to them on the road (provided you had internet access.) You would still have access to them on a faster external hard drive at home, but in a pinch on the road, you could fall back on the cloud.
Personally, I am not interested in storing data offline on the internet someplace, but it does seem to be all the rage these days. ISTR that some cloud servers' software (was it Microsoft in Windows 8?) can automate syncing a folder or drive. If that's correct, once you set it up, everytime you added/modified a file it would auto-synch with the copy in the cloud.