With the sudden ubiquity of data "in the cloud", or accessible from anywhere, it is becoming less and less relevant to store data on USB drives. One could even say that USB drives are becoming increasingly antiquated. However, until data I/O speeds in the cloud become faster, and until cloud security improves, people will continue to use USB drives to transfer data from one device to another.
USB drives come with their own security risks and shortcomings, however. Be aware of these risks before you use a USB flash drive in your workflow. If you currently use USB flash drives, familiarize yourself with some of the shortcomings.
Fake USB Drives
Watch out for people and businesses who sell fake USB drives. These USB drives work fine, but they do not store as much data as marketed. If, for instance, you buy a 4GB USB drive, a fake one would show 4 GB of free space on your computer, but if you try to add more than its actual limit (2 GB, for instance), the drive fails to write.
To avoid this, you can run a fake USB detection application, which will determine whether or not a USB drive can actually store as much data as it claims to be able to store. Be careful when buying USB drives online, and be aware that you run the risk of paying for more storage which you will not actually receive. It is best to buy USB drives from reputable dealers.
Flash memory, such as SSDs in MP3 players and laptops, fail after a certain number of read/write attempts. As a result, all flash memory eventually fails. Do not store anything on a USB drive which you cannot afford to lose, and be sure to back up all of your data.
When flash memory fails, it can be difficult or even impossible to retrieve. This is in stark contrast to hard drives, which still retain memory even when they fail. Further, there is little that you as a user can do to prevent your USB drive from failing -- it is a technological issue, not a user issue.
Bear in mind that USB space is *much* more expensive than cloud-based space. Only buy flash drives if there is no other way around it. It can be much more cost-effective to send data via e-mail, Dropbox, or LAN. While flash memory is becoming cheaper, it is still prohibitively expensive for large quantities of data.
Remember that a USB drive is only as secure as the person transporting it wants it to be. There is no such thing as "perfect security", and even an encrypted USB drive, in the wrong hands, can be dangerous. Transport as little sensitive information on USB drives as possible, and be sure to encrypt them if it is absolutely necessary. For example, when transporting tax forms to your accountant via USB stick, encrypt all forms into password-protected .zip files. Treat USB drives as carefully as you would physical boxes of files.
If you bear these risks and shortcomings in mind, you will be able to mitigate the negative aspects of USB drives and focus on the positive aspects: portability, universal read-write access, and ease of use.