LRC On Mac not backing up Catalog anymore

[photogaston]

Sonoma MacOS & Lightroom Classic (Latest Version)

Recently I bought an external SSD and moved all my photos to that drive. My Catalog too.
I want my LR backup to be on the external drive., I want to free up my computer from any large file systems. I do not care about having the backup and the catalog in the same drive, as it is backed up online daily. If I lose the drive, I still have online access. 

That drive is synced online with dropbox.

Now I am trying to backup my catalog and I get this error:

"Please check your folder permissions, and make sure that you have available space on  your backup drive and main catalog's drive."

To make sure the external drive was not the issue, I changed my backup folder location to my laptop's SSD. I still get the same error. 

I  also created a new catalog and imported the old one,  still get the same error. 

What else can I do?

[photogaston]

I also created a new catalog with 1 photo as a test, and iI can put the backup and the catalog in the same folder, in the external drive. Not sure what is going on then. 

[JohanElzenga]

I also created a new catalog with 1 photo as a test, and iI can put the backup and the catalog in the same folder, in the external drive. Not sure what is going on then. 

By @photogaston

A new catalog with just one photo is obviously going to be much smaller than you main catalog, so double check the available space on both drives, not only the backup drive.

[photogaston]

my external drive has 3.5 TB of free space

[JohanElzenga]

my external drive has 3.5 TB of free space

By @photogaston

What about your internal drive?

[JohanElzenga]

If your internal drive has enough space too, then most likely this is catalog corruption. There are other reports about the same problem, that turned out to be corruption. Often this can be solved by importing the corrupted catalog into a new one, but if I understand you correctly, then you already tried that. In that case your only hope may be a catalog backup from before this started to happen.

[photogaston]

You understood correctly.

Importing the catalog resulted in the same error. 

Is there a way to export the catalog as a copy? 

[photogaston]

I solved the problem, but I do not understand what's the relation. 

I freed up an additional 150 GB from my internal drive, now it sits at around 240 GB.

I backed up the catalog, and I got no error. Somehow you still need a certain amount of space in the internal drive, even if the drive is not used as storage?

[JohanElzenga]

Yes, you do. The Lightroom Classic application runs on your internal drive and it uses drive space for things like swap files (contents of RAM memory that is temporarily written to disk). 

[photogaston]

im not at home, but should have around 100 GB free space

[dj_paige]

Please check your folder permissions, and make sure that you have available space on  your backup drive and main catalog's drive.

I have gotten that message in certain situations, and restarting the computer fixed it. Also, it may be that somehow DropBox is temporarily preventing the backup, although I couldn't really say.

[photogaston]

The newer catalog backups without issue, howevrr I will try that later tonight.