USB flash drives and USB external hard drives are both good to have with your files on them.
I like having a USB flash drive that I can more easily take with me when I need to bring files with me and it's easy to just pop the USB flash drive into the USB port of any computer that I need to work with.
But the USB flash drives do eventually go bad after about 10 years as they lose the electrical charge over that time that helps to store the files and photos on them.
So if you have important data that you need to keep for a long time or forever you need to have the data stored on not only one USB flash drive but also a few others as well as an external hard drive, computer hard drive and in the cloud through Google Cloud Storage or other storage.
I also recommend having the files stored in Google Cloud Storage as well because in the event you have a house fire or other loss of your house and you cannot get the hard drives out of the house or your computer gets burned up or destroyed then you would still have the information.
I keep a copy of my files on USB flash drives in my cars trunk as well so that I have backups of my files that I need.