27 imac Late 2012 HDD Failure The HDD on my 27 imac Late 2012 has died. Is it replaceable? 
At present, I switch on (soundless) and get the white screen which loads up one third and then freezes. I tried Disk utilities which told me my HDD was inaccessible and un-repairable. 
If I fit a replacement HDD will I be able to restart, revive, reuse my computer?
Yes.
The HDD in my fusion drive in my personal 2012 iMac died.  I decided to open the iMac (remove the screen) and replace the internal spinning drive with a Samsung 850 SSD to improve performance.
The job will take about 90 to 120 mins total.  Most of the time will be spent separating the glue between the aluminum housing and the glass.  Once you separate it, you can easily remove the glass and access to the drives are extremely easy.
iFixit.com has excellent instructions on how to do this.  A couple of items you will need are:

Pentalobe Screwdriver
Replacement Adhesive Tape
3.5&quot; drive bracket

iFixit will recommend that you use one of those special tools to cut the adhesive.  I used a super sharp Exacto knife and wrapped some blue tape around the blade so that only a 1/2&quot; of the knife was exposed.  I cut all around all three sides then used a plastic putty knife to pry it loose.
It is much, much easier than it sounds.  Read the iFixit guide 3 times carefully before starting.  It will make it go much easier.
One thing to note is that in my particular machine, what I thought was an Fusion Drive was actually two drives put together (a 128GB SST and a 1TB HDD) using CoreStorage (a DIY Fusion Drive).   I added the Samsung SSD to the one already there, combined them into one volume and I have a 1.12 TB HDD
If you can boot your Mac, issue the command diskutil list and if you get anything that appears to be 3 drives in total, you have a DIY Fusion drive.
If you can't boot, you will need to take a look at the SSD connector on the logic board (it's by the fan).