1. Laptop power cord Panic. That is the feeling that hits you when you arrive at a destination and realize you forgot to pack the power cord for your laptop. Without it, you have mere hours of productivity before your laptop will die, and you'll be dead in the water. If possible, you should have a spare power cord that is always in your laptop bag so you don't run the risk of forgetting to bring one. If you can swap out the battery on your laptop, you might want to carry a spare battery as well.
2. Smartphone / tablet charger Along those same lines, you also don't want to embark on a business trip without the adapter and/or cable you need to recharge your smartphone or tablet. Ideally, you should have a backup always stored in your laptop bag for the same reason cited above.
3. USB flash drive USB flash drives seem like they're already on their way to obsolescence, but they still come in handy in a pinch. If a customer or vendor needs to share a file with you, or vice versa, a USB flash drive is often the most convenient and efficient means available.
4. Portable hard driveFor laptops with smaller SSD storage, an external USB hard drivemay be necessary just to store and transport data you need without filling up the storage on your laptop. It's a good idea to have a portable hard drive in your laptop bag in general, though. You can use it to carry the crucial software you rely on in case you need to reinstall it, or you may need to back up your laptop or store a file that's simply too large for a USB flash drive.
5. Ethernet cable There are times when you're traveling that an Ethernet cable comes in handy. Most hotels offer wireless network access, but it's often flaky and unreliable, and some hotels have wired Internet access, but no wireless network. You also may need to connect to a network at a customer or vendor site. An Ethernet cable is inexpensive, and could be a lifesaver when traveling.