I'm on 10.9.5, on a late 2009 27" iMac i7. A problem has developed a few weeks ago, wherein on occasion I'll lose the ability to Copy & Paste. It doesn't matter what I try - it doesn't work: (1) I right click on my mouse and select Copy and then Paste the item - nothing happens; (2) I use the keyboard to Command + C then I go and Command + V and nothing happens; (3) I select "Copy" from the Edit menu, then go and select "Paste" from the Edit menu and nothing happens. Basically, it seems like the Copy & Paste function doesn't work. I reboot the iMac, and the problem goes away. But after a few hours, it's back on again, and again, I have to reboot to correct it. Repairing permissions doesn't help. The HDD on my iMac is 1TB and it shows about 282GB used and 717GB free, I have 8GB RAM and it shows at least 1.79GB free. Any ideas as to what might be the problem here? Is it OS related, or hardware related? Would upgrading to 10.11 help? TIA!