There's rarely a safe and convenient place to put a portable external hard drive when using a laptop in a mobile, non-desk situation.
Example: You're working on your laptop and want to back up some files or work on some media that you have stored on an external drive. You pull it out of your bag and look around. If you're on a plane or train, at a coffee shop with a teensy-tiny bistro table, or lounging outside on a patio, you'll quickly realize there's no convenient place to put the external drive and be assured that while in operation it will be safe from liquid spills, getting whacked, or falling off of your lap or miniature workspace.
Adhesive-backed velcro strips.
A Geek Factor of 6/10 - This isn't for the fashion-conscious or those wishing to keep their laptop un-customized.
WHAT YOU NEED TO DO THIS:
# 15 minutes
# Velco or other brand hook-and-loop adhesive-backed fastener strips. (comes in rolls or pre-cut lengths)
# Paper towel or clean cloth.
# Scissors (with which to cut the velcro to size)
# Pencil (to mark and measure the velcro)
# Isopropyl Rubbing Alcohol or other grease-removing, residue-free cleaning agent.
# Portable External Hard Drive (or WiFi / wireless dongle, CD-Rom, tethered smartphone, etc)
New Material Cost: Under $10
Step 1: Velcro Engineering (Or, Is My Hard Drive Safe?)
The first and most immediate concern: is velcro strong and reliable enough to hold an external drive and weather normal movement from the laptop without the risk of it falling off?
Short Answer: Yes.
The Long Answer: Hook-and-loop products are rated by 3 important characteristics: Pull-apart strength, shear strength, and cycle-life. The products also come in two main grades: consumer-grade and "industrial strength" or "mil-spec" (i.e. military spec, or other governing-body specified and regulated).
Pull-apart strength measures how much force per unit area of velcro it takes to separate two pieces straight apart from each other. This is usually anywhere from 0.8 - 2 psi for both versions.
Shear strength relates to how much it takes to "slide" the opposing velcro pieces apart. It typically varies from 6 - 10 psi for most.
Cycle-life determines how many times the pull-apart / stick-back-together operation can be performed before the holding strength of the velcro degrades to 50% of its initial value.
This is where there is the biggest distinction between the two types. Consumer-grade velcro that can be purchased from any number of general hardware and goods retailers usually has a cycle-life of a few hundred. Mil-spec has a cycle-life ranging into the several thousands.
Some quick numbers:
Discussed further in Steps 3 and 6, I chose to have two strips of velco on the hard-drive intersect perpendicularly with two strips on the laptop.
Thus, there are 4 points of contact between the hard-drive body and laptop, and given 5/8" wide velcro strips, the total area of velcro contact is 1.56 in^2. Taking the lowest strength values (0.8 psi pull-apart, and 6 psi shear), this amount of velcro is therefore capable of withstanding 1.25 lbs of pull-apart force, and 9.3 lbs in the vertical, shear direction.
Most 2.5" portable external hard drives weight about 0.25 lb. This means that in the pull-apart direction, the HDD could withstand 5 G's of acceleration before pulling off, and up to 37 G's before shearing off in the vertical direction.
Likely, for your HDD and laptop to experience those accelerations, it would only occur when they slammed on the floor after having been dropped from a height of several feet. Normal use and fidgeting shouldn't be nearly enough to separate them.
The Usual Disclaimer:
Use your common sense when approaching this project. I've calculated and real-world tested using the above specs; they're solid. If in your own application of this 'ible you deviate significantly from the given advice by using too little velcro or attempt to attach an abnormally heavy hard-drive or peripheral, you are responsible for what may come of it.