It is not uncommon that a file system on a connected drive becomes damaged/inconsistent. That may be a possible cause of the behavior you see.
Prevent file system damage
To avoid any file system corruption, be very careful when you remove the drive. You should never just plug it out. Rather, first eject it with software. In Ubuntu, right-click the connected USB drive, and select "Safely remove drive". Wait a few seconds. You will get a notification that says drive removal is safe. Unplug the drive only after having seen this notification.
Alternatively, wait with removing the USB drive until the system has shut down fully.
Repairing a damaged file system
You can repair a damaged file system with the utility Disks or using the terminal. The file system first must be unmounted before proceeding with the check and repair.
- Open the utility "Disks"
- In the left pane, click your USB drive
- In the graphic map labeled "Volumes", make sure the relevant partition is highlighted. For USB sticks, you typically will see just a single partition. However, for larger drives with multiple partitions, you will see all of these in the graphical map.
- Click the "Stop" button (■). This unmounts the partition.
- Click the cog icon (third from the stop button) and select the menu item "Check file system". If this reports errors, you know you need to repair the file system. Then proceed with 6.
- Click the cog icon again, but this time select "Repair file system". You get a warning first and a message that it can take some time.
Final note
If you are careful mounting and unmounting the drive, file system issues should be rare. If, despite precautions, you regularly continue to face file system corruption, then a faulty USB drive or other hardware issues (even a loose cable) may be at the origin.