I LOVE my current Mountain Lion but my husband is nagging me about security issues using an older OS. My iMac shipped with ML and I have not upgraded the OS in the time I have had it. My hardware is compatible for Sierra: late 2013 iMac, 32 GB 1600 MHzDDR3 but this machine has a 3TB 7200rpm rotational disc. I do not want to upgrade the hard drive to an SSD because I have a lot of Adobe perpetual licence software and uninstalling and reinstalling Adobe has never, ever gone well in the past. If I had to do a clean install on a new SSD drive I would very likely lose some very expensive software. So I will be doing an upgrade on the current hard drive. My question is (I'm sure you were wondering when I will get around to it) is how slow will Sierra be on this rotational drive compared to my current OS. Am I going to be getting the dreaded beach ball a lot? This is a bit of a deal breaker for me and if I am going to end up with a machine that slows to a crawl I will just stay with ML, security risks be-damned. So anyone out there who has installed Sierra on a rotational disk willing to wade in here with personal experience please?