Internal boot drive read only

[imrazor]

A while ago I bought a 2017 5K 27" Retina iMac. The machine was in pretty good shape, save for a lone stuck pixel. However after a few weeks the Fusion Drive (128GB SSD + 2TB spinner) started having issues. I broke the Fusion Drive pretty easily, but was unhappy using a mechanical hard drive as my main drive. USB drives also proved to be too slow or glitchy, but a 1TB Thunderbolt SSD worked flawlessly.

However I am now starting to run out of space on the external drive. I thought I could just plonk some files on the old internal 2TB mechanical boot drive, but I got a rude awakening. Apparently it is read only for reasons I cannot determine.

How can I get access to the old boot drive? If possible I'd like to keep it bootable for emergencies.

EDIT: OS is Ventura 13.6.9, internal hard drive is APFS.

 

[Bigwaff]

1TB Thunderbolt SSD is boot drive w/ Ventura 13.6.9, correct? Internal 2TB HDD is original HDD which had been boot drive w/ which version of macOS? Was internal 2TB HDD erased and reformatted APFS while booted from external SSD so it is no longer a boot drive?

 

[imrazor]

Original specifications:

iMac "Core i5" 3.8 27" (5K, Mid-2017) Specs (Retina 5K, Mid-2017, MNED2LL/A, iMac18,3, A1419, 3070): EveryMac.com

Technical specifications for the iMac "Core i5" 3.8 27" (5K, Mid-2017). Dates sold, processor type, memory info, hard drive details, price and more.

everymac.com

The iMac has two internal drives, a 128GB Apple SSD and a 2TB mechanical hard drive. Both internal drives have been reformatted to APFS and are bootable; the 2TB mechanical drive also has a ~600GB Bootcamp partition. Both internal drives had Ventura installed on them, but they are probably a few updates behind the Thunderbolt install of Ventura.

When installing Ventura to the Thunderbolt drive, the installer complained that there was not enough room on the internal 128GB SSD which rather confused me. After freeing a few gigabytes, I was able to install Ventura to the external drive and boot off it.

I also tried copying a few small files to the internal SSD, which went fine. Not really sure what's going on here.

 

[Fishrrman]

"When installing Ventura to the Thunderbolt drive, the installer complained that there was not enough room on the internal 128GB SSD which rather confused me. After freeing a few gigabytes, I was able to install Ventura to the external drive and boot off it."

I recall reading that -- when compared to older, Intel-based versions of the OS -- modern Apple Silicon OS installs require a VERY large amount of "free space" in which to work. Seems around 60gb+/- is required.

I know that sounds ridiculous, but I believe that's the way things are now.
I welcome correction from others "in the know"...