and then emptied the "Trash"

Which kind of empty? Vanilla empty, which just erases the file's directory information, or secure empty, which (at least in theory) wipes the relevant disk space?

You can go to your Applications folder > Utilities directory and use Disk Utility to see how each partition is using its space. Not a solution, but may offer insight. Look into your settings also as Catalina has different everything for backups.

Years ago I used Parallels to translate old work files. It was temporary, maybe 4 months. Since then when I needed Win for some work, I got a low end PC just to keep them apart. The Win install in its own compartment on Mac for using with Parallels was a resource hog. Don't forget that it is as vulnerable installed on Mac as it is on a PC so be sure you have a current AV to keep out the nasty things. Not saying that is the cause, but it would be the first place I'd be looking.

I also have a late 2013 MacBookPro Retina with even less storage but I only use it for a portable device as needed. It is handy for camera work in the field. I did not update it to Catalina because after updating my desktop to Catalina I have had many more issues and old reliable tools do not work any more, some have no newer version and Cat is strict. Have you checked to see that you have no additional updates for WIN10 or Parallels? I am sure your new drive will make a difference. It sound like you're pushing limits on resources overall.

Try an app like Disk Inventory X, it can guide you to whatever's filling up the disk. Then you can do a more targeted search for a solution.

Sounds like system log files. If MacOS behaves the same as other *nix based systems, the OS reserves 5 to 10% of the disk space for privileged processes. When you as a user get an "insufficient space message, the critical processes can still write their data. As soon as you move user data from a full disk to USB, the system allocates the free space directly for use by the privileged processes.

Sounds like system log files. If MacOS behaves the same as other *nix based systems, the OS reserves 5 to 10% of the disk space for privileged processes.

This makes me think of the “Classic” OS, in which each program (I don’t think they were called Applications yet) had its own memory allotment, which you could change to taste. In time, I learned that some programs shouldn't be allotted too much memory, because the system itself needed a certain amount for things like saving or printing.

Thanks, Lammert:
How can I get some disk space back. I have literally moved gigabytes of user data off the the disk to make room for other data but now there is no room to put anything on the disk. Currently there is only 20mb free!

Thanks

Does time machine create backup files on the computer hard disk if the external time machine device is not connected? if so, is there a way to delete/stop that?

Your personal log files are stored in ~/Library/Logs. You can safely delete them, but don't delete the folder itself. Logfiles will be re-created automatically when they are needed.

The global application log files are in /Library/Logs and the system log files in /var/log. For these latter two groups of log files, it is probably better to clear them with the Console app than directly delete the files.

I wonder if it's Windows 10 that's filling the disk?

There is 1.8 Mb in the /Library/Logs folder... not much to be gained there
Using Disk Inventory X, I see the following:
33 Gb in "Applications"
- Most of that is in Adobe Photoshop and the MS Office Apps
- (I have Office for Mac loaded and I have noticed system performance problems after using Office applications, especially PowerPoint)
27 Gb in "Library"
- 19 Gb of that is in a folder called "Updates"
- Is any of that deletable
20 Gb in "Users"
- Virtually all of that is in my folder and 7 Gb is in music
15 Gb in "System"

The issue is not so much the static file (files that have been loaded) but the fact that something is consuming all the space I open up by deleting files as soon as I delete them. There is some dynamic function which keeps writing disk space?

[edited by: SeanF at 12:34 pm (utc) on Feb 2, 2021]

engine:
Windows 10 is on a different partition (100 Gb dedicated to Parallels)

You can use the following command on the command line to find the space usage of all files and directories. This command should also find any hidden files and directories.

du -h -d 1 /

@SeanF, just an idea as I remember Microsoft talking about one of its updates requiring much more allocated space.

33 Gb in "Applications"
27 Gb in "Library"
20 Gb in "Users"
15 Gb in "System"

Adds up to a mere 95 GB, out of 150 GB. Weird.

Drive is healthy?

Take look into /System/Library/Caches/
IME, The usual suspect is com.apple.coresymbolicationd
I've managed to free-up ~ 20% space with sudo rm -rf /System/Library/Caches/com.apple.coresymbolicationd

Good luck

SeanF, did you manage to resolve this?