Using an external hard drive to update Macbook Air?

I have several thoughts to share which may be helpful.

• First, if your available storage gets too full, your computer will stop functioning and that is a real problem.
• To get the most accurate picture of how much storage is used/available, you should use Disk Utility. Other methods are not accurate.
• Have you emptied your Trash bin? If not, doing so will reclaim some storage.
• Your solution is to attach an external disk and copy files onto it. Especially files such as videos, photos, etc. Then they can be erased from your HD to clear needed space.
• If used only for storage, the disk can be a lower cost mechanical disk rather than a higher cost SSD. It can be easily connected using a vendor provided USB cable.
• Before being used, the external disk may need to be first reformatted. ONLY use Disk Utility for this. Do not use any maintenance app provided by the disk vendor. It should be trashed. They only cause problems in Macs.
• Once done, you can then update or upgrade your Mac's operating system. When completed, be sure to then move the installation file from the Applications folder to the Trash and empty it to clear storage space.

well, exactly how much space do you have (total) and how much is available? An external will only help if you move some of your files to the external and then delete the items from your disk creating more space.