Harry Cover wrote:
My startup disk's “User > Documents” folder is getting too big.
To save some space on my startup disk, I would like to move it to an external hard drive, and create a link in my startup disk, pointing to that moved "Documents" folder.
Question:
Which kind of link would be better: Alias, Symlink or Hard link?
Thanks in advance for your advices.
Documents is baked in directory— I would move content of the directory itself so as not complicate the Finder defaults.
typically photos, videos take up much more storage space than txt documents(?)
for comparison: Move your Photos library to save space on your Mac - Apple ...
from the Terminal App you can get a good look at directory size, copy and paste:
sudo du -h -d 1 ~/
ex.

See if there is any insight here—
How to free up storage space on your Mac - Apple Support
Free up storage space on your Mac - Apple Support
See used and available storage space on your Mac
See used and available storage space on your Mac - Apple Support
User tip: "Other and What Can I Do About It ?"
What is "Other" and What Can I Do About I… - Apple Community
Try something like OmniDiskSweeper for a GUI to get a good look at itemized file size and location:
OmniDiskSweeper http://www.omnigroup.com/more