I have a relatively new iMac (about 15 months old) at the top spec, with 3Ghz i7, 32Gb Ram etc etc.
Plugged into that are several USB 3 drives.
Sometimes, I can hear one of those drives spinning up and whilst it does so, I get the old beachball.
This lasts perhaps 5 seconds during which I can do nothing with the Mac.
It happens at times unrelated to the use of the drives - for example, today I opened a pdf attachement in an email and tried to scroll down. Suddenly - whir whir and beachball.
Anyone know why?