Beachballing and "disk0s10: I/O errors" after upgrading to Snow Leopard
Here's my situation: I recently upgraded my iMac (2.93 Ghz Intel Core Duo, 4GB of ram, model id is 9.1) from Leopard to Snow Leopard. I did a basic install (not a clean install), the installation went smoothly, no errors were reported and in less than 40 minutes I was up running Snow Leopard.
But... immediately, on the first launch, after a couple of minutes my Mac started beachballing, leaving it completely unresponsive for like 5 minutes. Afterwards, the spinning pinwheel disappears, my Mac continues running normally, and I don't experience much issues. At first I thought it was a one time thing, probably saving some settings up after the first launch, but... it has been happening every time I start my computer! Every time, it starts up, all my startup programs launch, the desktop icons appear, and then suddenly after a couple of minutes it starts beachballing again.
I now start to suspect there's something wrong. I launch the Console and take a look at the logs. There at the exact same time the beachballing occurs, i find like 3 or 4 subsequent "disk0s10: I/O errors". Now I'm starting to think it's a hard disk issue, so I bring up Disk Utility and check the S.M.A.R.T., it says everything is OK. I verify the disk, and it says everything is OK. Now I don't know what to suspect.
Something tells me it's not the hard disk failing (maybe it's denial), because the problem started happening immediately after installing Snow Leopard. With Leopard I didn't have any problems at all. Also, once the beachballing stops, my computer runs smooth and fast, much faster than with Leopard; and it seems the beachballing and the errors don't happen again until the next restart (though I haven't yet used it for a long period of time since this problem happened like three days ago). Another thing is that I recently transfered like 30GBs worth of files and I didn't receive a single error. Everything seems (as for now) to run perfectly well after the initial beachballing occurs.
So, I'm now facing two options: (1) Make a clean install and see if the problem persists, or (2) Replace the hard disk.
As all of you know, both options are pretty time consuming and annoying, so I really want to troubleshoot everything I can to see if the problem has a simpler solution. What other things can I check? Have any of you had this problem before?
Sorry to make my post so long but I believe the more information you get first hand the better, right? Please, I will really appreciate any advice.
iMac, Mac OS X (10.6.7)