'Your startup disk is almost full' Mac Error
Users who don't regularly do manual maintenance on their Mac computers might eventually run into a relatively uncommon error message. A lot of people tend to get scared when they see the Mac error "Your startup disk is almost full".
Obviously, your startup disk is your drive where your operating system is stored and runs off of. You do not want that drive to be filled to the brim with data as that may cause slowdowns in your Mac's operation and may even lead to occasional freezes and hang-ups, in extreme cases.
Assuming a default install, your startup disk drive will be named Macintosh HD. This is the disk drive where the operating system will store all your files, settings and applications by default as well. This means that a hard drive that is a bit older and a bit smaller than what some modern software might require can get filled to the very brim without too much difficulty.
With the deployment of the Big Sur update for Apple's operating system, the notification text as well as the message popup window for the error have been tweaked a little. The new message reads "Warning! Your Mac is running low on disk space. Please free up some space by removing unnecessary files or changing your cache settings. Free available storage: [space here]"
Here's a list of things you can do to free up space on your Mac. Some of those tips may seem obvious to some, others are probably not so obvious, but all will help.
Clean your Trash
Sometimes users can forget that deleting an item, application or a file does not free up the disk space it occupied completely by default. Deleted files and apps are sent to the Trash, but they still take up valuable drive space, because the Trash serves as a temporary file purgatory and items can be recovered from it.
If you have been deleting files for a couple of months and have not bothered to check your Trash or empty it, going through the items in the Trash and cleaning selectively or simply emptying everything will usually free up quite a bit of space on your drive.
Clean your Downloads folder
Somewhat similar to the Trash, your Downloads folder probably isn't getting enough attention. A lot of users are very "fire and forget" with their downloads, often downloading chunky media files and app installers and leaving them in there after a single look or after installing them.
You would be surprised how quickly the Downloads folder can grow in size. That's why it's advisable to take a trip to it at least once a month and scan through what you have downloaded. Make sure you delete any large files you no longer need and then remember step one, meaning you would also need to clean your Trash to reclaim that disk space.
Clear application caches
Almost every app on your Mac generates cache files or some sort, to speed up its work and give you a smoother, faster experience. In almost all those cases, this happens at the expense of disk space used to store the caches content for faster access.
While the convenience of certain apps booting a little faster or your web browser loading some pages a little quicker, it is still a good idea to give your cache a scrub every few months. This is especially true with smaller hard drives that can run into the "Your startup disk is almost full" error.
To clear your caches, open Finder, then use Go to Folder. In the dialog box, enter ~/Library/Caches and click Go.
It is generally recommended not to delete the separate folders you will find inside this location. Rather than deleting the folders themselves, go into each of them in order and delete the files stored inside them. Don't forget to empty your Trash afterwards.
There are a couple of other ways to trim temporary and infrequently used files on your Mac, but in the vast majority of cases, those files are so small and their footprint on your hard drive is so insignificant, that deleting them will not accomplish much. However, just those three steps above will often help you reclaim at least a few gigabytes of space, which should give your MacOS enough space to breathe and work without issues on your system hard drive.