Q: Occasional Delay on Boot

Hey,

Ever since updating to Lion about 2 years ago, my macbook pro will occasionally have a weird delay upon start up. Normally, when I hit the power button, I hear the disk drive do its normal "buzz" thing and the apple logo immediately pops up. I log in and it boots, no problem. However, on rare occasions I'll turn it on and there will be about a 3-4 second delay between the disk drive buzz and the logo appearing with the chime (usually there's no chime because I rarely have my speakers turned up). I'll log in but it will never boot entirely. The little wheel will just spin in the center of the screen indefinitely. I've let it sit like this for 15 minutes before. What I end up having to do it force it to shut down. It will then restart normally.

This failure to log in happens only when the delay between the disk drive buzz and the logo appearing occurs. I've learned to never try to log in when the delay occurs. If it does, I click the "restart" button at the login screen and wait for it to restart without the delay between buzz and logo. I recently got into the habit of just putting my computer to sleep when I'm not using it, instead of shutting it down every night. I restart it once every few days.

This seems to have started after I updated to Lion way back when and the "bug" has carried over to each OS X update ever since.

My question is, why does this happen?

Thanks!

Macbook Pro 13 in, Early 2011, Dual Core i5 2.3 GHz, 4 gb RAM, 128 gb Intel SSD, OS X v. 10.9.4