Can't backup iPhone because no storage space left when there is?

I am trying to backup my iPhone but iTunes complains that I do not have enough free space on my computer to do so.

I don’t believe this is correct however, on my machine I have my Macintosh SSD (which has 90GB remaining), and an additional drive which has 300+GB remaining.

Many years ago I changed my MobileSync directory to a folder in my additional drive as there is a lot more free storage on there. I did this using a Symlink (article here covers how to do so), and it has been working as I can see all the existing backups up until now all stored in my second drive.

I am trying to do a backup of my iPhone now which I believe has around 140GB worth of data to backup, but I get the following error:

I’m not sure why its showing this error because there is no way backup is larger than 300GB, which is how much free storage I have on my second drive (where my backups are stored).

I can confirm the symlink is still there, as when I navigate to the MobileSync folder it directs to my folder made in the second drive.