I used to carry a USB drive containing important files on my keychain. Just in case the drive fell into criminal hands, I encrypted the files. Unfortunately, the drive's software, which I needed to access the files, didn't always work the way it was supposed to. So I gave up.
Thankfully, you don't need to carry a keychain-based USB drive anymore. All you need is a decent app to transfer files between your computer and smartphone (or tablet).
Apple's iOS devices were apparently out to lunch when file manager interfaces were handed out. So I gave the recently released iOS app, Pocket Drive, a try. The file manager isn't perfect, but it gets the job done.
Pocket Drive is free, though the no-cost version limits you to just 512MB of storage. You can upgrade to 4GB ($2), 16GB ($3), 32GB ($4) or unlimited storage ($5). Those costs are one-time fees and are not recurring. You'll probably want to upgrade because 512MB isn't a lot of space. The app also incessantly nags you until you cough up a few dollars, but at least the free version doesn't annoy you with ads, as the similar, free File Manager app does. (A dollar upgrade removes File Manager's obnoxious ads.)
Pocket Drive requires very little set up. When you launch the app for the first time, your computer automatically detects your mobile device. After installing Pocket Drive, my iMac listed my iPhone 6 Plus among my "Shared" devices. I clicked on the iPhone in the Mac OS X Finder to establish a wireless connection and tapped to accept the connection (a security measure).
With the connection established, I dragged and dropped PDFs, Excel, Word, video and music files from my iMac's hard drive to my iPhone. Most of the files showed up immediately in Pocket Drive, though a 166MB video clip understandably took a few seconds. When they had transferred, I could tap to view any of the files; Pocket Drive supports video and MP3 playback. In fact, the MP3 player is pretty cool, because it mimics a record player. You can also connect Pocket Drive to your TV to play videos using Apple's AirPlay and an Apple TV.
Pocket Drive's "Share" icon, lets you open files in other iOS apps for further processing. For instance, I opened an Excel spreadsheet in the Excel iOS app, saved it as a new file in a Dropbox folder and made a few edits. The process was simple and painless.
It's not quite as easy to move files from your iOS device to your Mac or Windows PC using Pocket Drive. One way to do this is to open a file in Dropbox or another cloud-storage account's iOS app, then choose Pocket Drive as an "Open In" option. However, this method won't work if you try to transfer photos from your iPhone's Camera Roll, because there's no "Open In" option. This isn't a deal killer, as Apple's AirDrop and iCloud's My Photo Stream, as well as Dropbox's free Carousel app, make it easy to upload pics to your Mac.
Pocket Drive isn't your only "virtual USB thumb drive" option. Along with the aforementioned File Manager, File Manager Pro ($5) also does the trick, though as of this writing, it was last updated nearly a year ago. Another option, FileMaster (free, with $3 in-app purchase to remove ads), which was last updated Dec. 6, 2014, has a 4.5-star user rating.
In short: Unless you need a drive handy to stick into someone else's computer, an app like Pocket Drive is probably all you need for portable storage.