My kids had the late 2009 version with 4 gb ram and and after I did the update from 10.10.2 to 10.10.3 it seems to stuck at the same loading sequence with the apple logo, no matter what I do!
The late 2009 has the original 160 gb harddrive and few months back I bought WD black 500 gb edition and it works great - however 10.10 is really slow on our late 2009 mac mini.
I had tried with the 160 gb and 500 gb there is no way I can get 10.10.3 to work.
So what I am thinking now - sell it or downgrade to 10.9 so it would work better.
Is there any guide to downgrade? I did try with the mavericks pack from apple, but I am not allowed to install it on the mac mini - wonder why!
My early 2009 MacBook w/4 GB RAM works well with OS X 10.10.3. Backup your important data to a cloud service or to an external drive and then do a clean install of OS X. If our earlier models work then yours should, in my opinion.
It seems everything went fine.. After the alt-cmd-p-r reset it seems to help - though this time I didn't force reboot after the yosemite install. Then I could easy install the 10.10.3 and my Mac Mini is even faster than before! It's booting up very fast (regard how slow it was before) - so I am quite happy! In fact I was 2 minutes from selling it!