Hey there @LuckWinner - how are you today?
Why don't use selective sync which is a great way to free up space by syncing only particular folders to a specific device instead?
Anyhow, if you choose to go with the external HDD option, take into account that Dropbox can indeed be used with an external hard drive as long as you can ensure the drive is always connected to your computer while Dropbox (i.e the desktop app) is running. If the drive is ever disconnected from the computer while Dropbox is running there is a small chance that the Dropbox software will start deleting files before realizing that the entire drive has been removed.
Additionally, you should ensure that the file system of the external drive meets the file system requirements. Those can be found in the following Help Center article:
To move your Dropbox folder to an external drive use the steps in the following Help Center article:
[The location you select is the folder the Dropbox folder will be moved to, rather than the folder your Dropbox files will be stored in.]
Let me know if you need anything else please!
... View more