Can you move documents folder to external drive... if so, how?

The Documents folder is part of OS X. Better to not try and move it to an external drive.

There are alternative ways of freeing up disk space.

Click the Apple  top left in your screen. From the drop down menu click About This Mac > More Info > Storage

Make sure there's at least 15% free disk space. OS X Tips Where did my Disk Space go?

Reindex Spotlight > Spotlight: How to re-index folders or volumes

Freeing space on your Mac OS X startup disk

Repair permissions.

Launch Disk Utility located in your Applications > Utilities folder.

Select the startup disk on the left then select the First Aid tab.

Click: Repair Disk Permissions

This can take awhile so don't be alarmed.

Restart your Mac after Disk Utility is finished.

To move a part of the Mac OS X system to an external drive, that is not permanent

to the computer itself & does not contain the rest of the OS X, is asking for trouble.

Before attempting to modify or change how the system works, you should backup

the entire content and not just by Time Machine file methods. You may need to

restore the entire content, and without a disk image or bootable clone copy of that

system as you have it, would not be as easy or directly possible to restore.

Some applications that save their copies to the system's document folder would

not likely find it on an external drive. Or when it fails to be identified correctly,

you may lose direct access to the items sent there.

I've seldom relied on the system's Documents folder, but instead have created files

for my own content in locations where I hope to retrieve them. Things happen, so

the backup and a backup of the other backup, is recommended even if you don't

try moving system folders to another drive, an external archive that may be lost.

Good luck & happy computing! 🙂