Hi Thanks for your reply HWTech,
That is my concern also re the HD Data volume ( I have included some photos of my system and space available. When my iMac was updated last I had Apple do it with me, only because of a virus scare I had, (it turned out that it wasn't a virus) I didn't want to update, I was happy with my iMac, but Apple suggested I jump over a couple of running systems, and update straight to what I have now (Big Sur 11.7.10)
When I updated my computer wouldn't even turn on again, so totally due to Apple's 'help'!
I was assured it would be fine to update that way, even though I questioned the jumping over 2 running systems straight to Big Sur. I have always read you should never miss out the next running system step.
Since then this 2 drive thing appeared after Apple helped me get my Mac turned back on and running, (this took 3 different people at Apple in Ireland, it took about a week to get it going again). This two drive thing was a result of getting my Mac running, I suppose it was a way of backing it up if it stopped working again. Apple said they would help me with that aspect when I was sure it was all working ok.
To be honest I just left it as it was because it was working and I didn't want to mess it up.
I haven't ever considered updating the running sytem since because Im worried it will stop working again.
So yes I reckon you could be right about the 2 drive thing? but I also have read here on Apple discussions that others also have a 2 drive thing with 'Big Sur'.
Maybe you could help me check the 2 drive thing, I'm not sure where to look, I had a MacBook Pro and had no trouble finding things on there! You will see I have included some photos of my disc space when I checked a couple of days ago. Since then I have taken a lot of files of my Mac to make some space. What are your thoughts on the word 'Purgeable" ? I really can only think it means stuff I need to get off my iMac ?
thanks for your help!