Storage Space used by OS Catalina

The Catalina installer is about 8GB - the actual installation, including the built in Apple Applications which have also been upgraded, takes about 20GB, the same as your Mojave. After downloading the Catalina installer you'll be asked to run it. The Catalina installer will spend a few minutes copying some files onto your hard drive and then reboot the computer. At restart you'll see a black screen with the Apple icon and a progress bar as the installation continues. Once it is finished (about 90 minutes) the computer will reboot once more and Catalina will be launched. All Mojave files will be removed/replaced and you'll noticed that your drive space has been reduced by 1 or 2 GB.

Note: Before you install Catalina make sure your computer is backed up. After the Catalina download is finished you can quit the installer. You'll find it in your Applications folder and can run it at a later date. For example, I downloaded the Catalina installer the day it became available but I quit it and didn't install it until Sunday.

I am a little confused: RAM has nothing at all to do with storage, it is random access memory used to run, launch, etc. the OS and your apps. And, it is configured to use all of the RAM available which is 8 GB. Your flash/hard drive is 128 GB and that is where all the action takes place and the OS and everything else is stored. FWIW, 128 is really, really small, you will need to keep an eye on it. Currently you have 87 GB, deduct 10 - 15 for empty space and you only have 70+ GB for storage.

Yes it sounds about right. 2 things: most of us are very suspicious of Clean My Mac. My own opinion is that it cleans your wallet and at best does nothing horrible but nothing useful to the computer. Also, you are right at the edge in terms of drive space. Apple recommends that you keep at least 10GB free for temporary files.

There's a program called OmniDiskSweeper that can help you find what's on your drive. Here's the link: OmniDiskSweeper At the Apple App Store there's a program I really like that does the same thing but in a friendly way. It's called DaisyDisk. Not free but well worth it.

A 128GB drive is pretty darn small. I don't know how people can function with that. I could barely function within the 256GB my first MBA had. My current MBA has a 512GB SSD and I'm okay with it but I stream my music, I don't keep any on the computer. Your first post suggests that this is a 2018 MBA and if so there's no upgrading the drive but if it is a 2017 model or older there are drive upgrades. You might also take a look at the mini SSDs. I have a Samsung T5, about the size of a credit card and about 3 card cards thick. Self powered. I use mine as a portable TimeMachine drive but it would be fairly convenient to travel with if you need external space.

Looking over our conversation I obviously misread your 87.11 GB available as 17.11GB available. Obviously you have lots of space on the MBA's internal drive. So you have an external drive that is low on space? Is it for TimeMachine backups or what? How are you using it.

No problem, glad to help.