Storage eaten up by "other" folder since upgrading to Catalina.

"Other" just means files that aren't tagged as "System", "Music", etc. That in itself would cause a bit of confusion because "What is it?" But a bug in Spotlight makes the issue worse. Files are designated as "System", "Music", etc and anything that doesn't fit is "Other". Okay. But over time the Spotlight index tends to lose tags so files that were once designated as a specific type become "Other" as well. So ignore "Other". You can attempt to rebuild the spotlight index which might help: https://support.apple.com/en-us/HT201716

Please don't be insulted by these suggestions - many people make these mistakes and we have no idea of a user's experience level. Make sure you empty the trash after deleting files. If you deleted pictures in the Photos app, you have to take an extra step to immediately remove them from your hard drive. Look in the Photos' sidebar for Recently Deleted and empty it. Photos keeps deleted files for about a month in case you change your mind.

Sometimes a cache file or error log doesn't get properly trimmed. If you restart in Safe Mode (restart and hold the shift key until you see the Apple icon) the restart will be slower than normal because the computer is performing a number of tasks it doesn't normally do at start up. One of those tasks is cleaning up caches and logs. Often times a safe restart frees up a significant amount of hard drive space and stops a mangled cache from growing ever larger.

Then again, with only 150GB for your Mac OS, it may be time for a larger SSD. That's pretty darn small these days.

Are you really losing space or are you just seeing "Other" getting bigger? On one of my computers after Spotlight's index "Other" will immediately begin growing again but free space is staying the same because "Documents", "Pictures", etc are shrinking. If I wasn't paying attention and aware of the issue all I'd notice is "Other" growing larger. In other words, you have a problem if every day your free space is getting smaller but you aren't adding content and applications to explain it. But if it is just "Other" getting bigger, that's not a problem.

Because Spotlight is so flakey I've invested in DaisyDisk (available on the App Store). A free program is OmniDiskSweeper. Both programs help you examine your hard drive's content by type without depending on the Spotlight index. I can recommend either - DD has a nicer interface but both do the job well.

Finally, EtreCheck is a diagnostic tool written by an ASC user that gives us a look at the system (no personal information is shared) and it is helpful to see what software is running and what's going on. It would be worth downloading, running, and returning here with the results. Click on the box indicated below to paste the results.

You may need to consider 'upgrade internal SSD storage capacity' to larger size.

And perhaps use external drive for additional purposes; including Windows

OS. See numbers available for total storage (to macOS, its applications) and

various limited-access partitions that are created on original drive w/ APFS.

Upgrade examples for various Macs can be found from mac-centric supplier

see what may be available; good ones guaranteed to match your hardware.

They also have selection of external storage which can boot another macOS.

A storage drive of at least 250GB (512GB better) should be minimal for Catalina.

(Solid state 'blade-type' internal drive options such as OWC 480GB to 1TB.)

{.My mid-2012 MacBook Pro (non-retina) 13-inch does OK with 250GB SSD and

16GB RAM; & Mojave macOS 10.14.6. Add external storage via fast connection.}

However daily use Mac desktop has dual internal hard drives; slower but sure.

An older Mac OS X with three systems installed. With Backups? ~ A priority!

Once you've a system on APFS, the Container would accept later macOS systems within separate partitions or sections. To sort their content may be wise to use tools suggested by dwb; and know how the system structure has now been built. Catalina is OK, if you're already prepared past transitional stage.

And limitations with newer format in how it uses capacities. The system controls more than previous; and as such is more safe and secure, once you understand the changes. Activity Monitor and other system utilities can help.

Since you've already been transitioning away from older 32-bit software and drivers, that's fine. If you've already invested in Catalina, and have only one Mac, that's kinda rough. I've had no need to go beyond Mojave, as that's where my older software can still work. Have five macs; three portables. A few considered functional antiques.

And content once in APFS, is handled differently; in fact could have from High Sierra on, its a different game.

But no reformat should be needed if you know what the content actually is; the EtreCheck utility is a good tool.

I'm guilty of assuming most users would already have that, part of their kit. Given value vs. cost 'omnisweeper'

is among the other known good useful management schemes and another keeper.

If you'd been using macOS since High Sierra, fair chance with SSD drives, its been automatically using APFS. Though you could choose between HFS+ journaled and APFS initially in High Sierra on rotational drives.

My MB/Pro (2012) hasn't seen much use with Mojave; though it could use 32-bit transitional software. And a Mac mini i7 server with older systems onboard. Plus a few working PPC G4 Macs with Tiger/Leopard; and early MacBook1.1 with Leopard. (TenFourFox browser works in early Intel-mac.)

I've not embraced latest, any more. Years ago, I had time and places to do more, and learned how to deal with tech. With other life chores and urban changes, more of that is offline. With exceptions; iPhone SE (iOS13.3.1) which continues to challenge conventional thinking and wastes time, those commodities I've no free amount to fritter. 'Touch-screen' doesn't really help.

Speaking of time, this lengthy 'reply' has taken a life-of-its-own!! Sorry about that.

Can't honestly say I'd edited it near enough. Down to 'just the facts'?!? Not! 🌻

The Time Machine is part of the macOS; because the system already makes temporary 'snapshots', on main drive, this second external drive dedicated to Time Machine saves them into each automatic daily archive. These get rotated out (copied over) as the dedicated external drive gets near capacity; so you have certain versions of the system in that archive, for a time.

My backups with older Mac OS and Mac OS X used to be Clones. On FireWire enclosures where bootable clones were king.

Still have some, and older Macs with last build G4 and early archives saved; you'd put chance in your favor, with more than one backup scheme implemented, and use second dedicated 1TB USB HDD, port-powered.

You can also have clone utilities make time-incremental full backups, too (such as Shirt Pocket's Super Duper or Bombich's Carbon Copy Cloner) each of these has different details that favor them. You can never have enough backups. Some at home via Wireless base (connect time machine USB there) and ones for the office, etc.

Anyway, I've not had to restore from backups; but have a few macs that have at least Time Machine ones on external drives; some also have Clones on drives in self-powered enclosures.

• Apple OS X and Time Machine Tips | by J. Pond (2013 archive)

http://oldtoad.net/pondini.org/OSX/Home.html

Suggestions include good names for backups, storage solutions, and duplicate efforts ahead of failure.

I have one big reason to be on Catalina: Sidecar lets me use my iPad as a second display when I’m on the road with my notebook. I’m spoiled by a 28” on my home computer. There are smaller benefits like improved Notes and Reminders. As I approach retirement I’m looking to replace power programs like Things (project management and reminders) with less demanding (and free) programs. But I could have easily remained with Mojave another year.

But there’s one big issue with returning to Mojave from Catalina. If you don’t have a pre-Catalina backup reverting isn’t going to be pleasant. You cannot back up Catalina, reformat, install Mojave and then use the Catalina backup to repopulate the computer. You’ll have to revert manually. But if you have a Mojave backup you should be okay. But your BootCamp installation may be need to be redone too so don’t start this adventure without a Windows backup as well. And don’t forget, TimeMachine doesn’t back up BootCamp.