Application Memory issue - what the...? Final Cut just stops...

Background rendering: Background rendering in Final Cut Pro for Mac - Apple Support

On modern machines it's usually unnecessary. If there's a section that can't be played in realtime, you can render just that section. These render file can get quite large.

Since we can't see your machine, I thought it was possible you had configured this render cache to go to your system drive, which could fill it up. Manage render files in Final Cut Pro for Mac - Apple Support

Regarding my question about movies in your Pictures folder, I (mis)understood you to mean that these movies were mysteriously appearing there and you were deleting them. I see that you just have a regular iPhoto library that happens to be very large. Luis and Tom's suggestion to move it to an external drive to free up space on your system drive sounds like a potential solution instead of installing a larger system drive.

Please post an EtreCheck report https://etrecheck.com giving the application Full Disk Access in Privacy & Security System Settings. Copy the report and use the Add Text button to paste the result into reply box. 

Free: 7.51 GB

your system drive is full to the point of catastrophic failure. Your drives need to have a minimum of 15-20% free space, about 200 GB.

The Additional Text button would allow you include lengthy, complete reports.

The OS requires swap space on the boot drive to do all sorts of things related and unrelated to FCP.

Can you describe the videos that are in your Pictures folder?

Do you have background rendering enabled in FCP prefs? If so, disable it.

The catastrophic failure warning is no joke. If you run your boot drive out of space, you can end up with a machine that won't boot and requires time and effort to resurrect.

That's pretty standard for pro apps. The applications move large amounts of data through system processes. As Terry said there is constant swapping and caching of files that's all done on the system drive. You really need to find a way to clear off free space on that system.

The Photos library does not have to be stored on the system drive. There are ways to do this, though I'm not familiar with it himself. You can probably find out in the Photos forum.

Tom Wolsky wrote:

The Photos library does not have to be stored on the system drive. There are ways to do this, though I'm not familiar with it himself. You can probably find out in the Photos forum.

That is very easy to do.

In fact the Photos library is a package (like, for example, each library in FCP).

Photos can use different libraries (except just one at a time, unlike FCP).

The user could simply:

1) use the Finder to copy it to an external drive,

2) start Photos holding down the Option key, and selecting the library in the external

3) In Photos->Settings, click the "Use as System Photo Library"

From then on, and until the user decides otherwise, Photos will automatically use that library in the external (and so, by the way, will FCP's Photos browser)

You’re missing the top section of the report with all the system and drive information. Copy the report using the copy function in the Etrecheck application.

As Tom and the Etrecheck report say:

Low disk space - This computer is running critically low on free hard drive space.