iMac 27" Late 2012 HDD seem to have died

[resting]

But some how I'm still running on the 120GB SSD I believe.
Disk space is filling up.
Should I replace the entire machine or just replace the HDD?

Late 2012 is the oldest model that Catalina supports.
Worried that this machine wouldn't be supported in the next OS release which would make the HDD replace a waste of money.

 

[DeltaMac]

Or, you can choose to continue to use a Mac that does what you need.
Even if the 2012 is not supported in next macOS release, you will then have about 3 years that Catalina would continue to get security updates -- gives you plenty of time to decide when you will move on.

 

[Fishrrman]

Did that 2012 iMac come with a fusion drive?
If so, what size?

Does it boot?
What does disk utility show you?

It sounds like it might have been a 1tb fusion drive, and the HDD portion isn't working or no longer recognized, and you're booting/running from the 120gb SSD?

Who set it up to be this way?

IF that's the case, I wouldn't bother replacing the internal HDD.
I'd just "let it sit there, dead-in-place". RIP

Then... I'd get an EXTERNAL USB3 drive (probably a USB3 SSD), plug that in, and use that for my "extra storage space".

Having that EXTERNAL drive will MAKE IT EASY when it's time to replace the old iMac with something new.

 

[mmomega]

Same happened to my 2012.
I replaced the HDD with an SSD, it's working great today still.

Mine was a Fusion, 1TB HDD+120GB SSD.
$19 for adhesive kit and $70-100 SSD is so much less for a working computer than $2,000 - $4,000

Just because Apple releases an OS upgrade does not mean you have to essentially spend several thousand dollars on 10.16 a year ahead of time.
Swap the HDD and take the next year to figure it out because you'll have from June 2020 at the soonest until October 2020 to know if you upgrade and your current computer can't be upgraded with 100% certainty.