Beachballing and "disk0s10: I/O errors" after upgrading to Snow Leopard

Question

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.

Answer 1

OK. Here's what I found out.
When I boot in Safe Mode the problem does not occur (and I discovered why, I'll explain further along). Also, Disk Utility didn't report any errors in either the "Macintosh HD" volume nor the "WINDOWS" volume.
Looking closer at the console errors I noticed a couple of things. Here are the console errors:
6/19/11 4:03:02 AM    kernel    disk0s1: I/O error.
6/19/11 4:03:02 AM    kernel   
6/19/11 4:03:02 AM    kernel   
6/19/11 4:04:09 AM    kernel    disk0s1: I/O error.
6/19/11 4:04:09 AM    kernel   
6/19/11 4:04:09 AM    kernel   
6/19/11 4:05:17 AM    kernel    disk0s1: I/O error.
6/19/11 4:05:17 AM    kernel   
6/19/11 4:05:17 AM    kernel   
6/19/11 4:05:17 AM    firmwaresyncd[63]    /sbin/mount exited with 71
6/19/11 4:05:17 AM    firmwaresyncd[63]    Command /sbin/mount output: mount_msdos: /dev/disk0s1 on /Volumes/firmwaresyncd.QsGgXz: Input/output error
First of all, the error is reported on disk0s1. So I ran diskutil list on Terminal to check which drive was this. These are the drives and partitions that were listed:
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *640.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            605.5 GB   disk0s2
   3:       Microsoft Basic Data WINDOWS                 34.3 GB    disk0s3
As you can see, disk0s1 is the EFI System Partition (ESP) in my Mac. After a little research I found out that this partition is used as a stage for handling firmware updates. I also found out that this partition is not really needed to run Mac OS X. This explains the subsequent appearance of the firmwaresyncd command which, according to the Apple's developer manuals, runs at boot time to synchronize the firmware files from the root filesystem to the EFI System Partition (ESP). Also, it does not run when the system is performing a Safe Boot.
Now this explains why the problem only happens after I boot and never happens again until I reboot. It also explains why it doesn't happen when I run in Safe Mode.
Therefore, the problem I'm experiencing is basically the following:
At boot time the firmwaresyncd command tries to access the EFI partition to synchronize the firmware files from the root filesystem to the EFI System Partition.
It seems that the EFI partition at disk0s1 is either corrupted or inaccesible so the firmwaresyncd command can't access the partition and after a couple of tries throws the input / output error.
Now the next thing I did was to run Single User Mode and run  fsck_msdos /dev/disk0s1 to see if I could repair this partition. Unfortunately, I got the following error:
:/ root# fsck_msdos /dev/disk0s1
** /dev/disk0s1
** Phase 1 - Preparing FAT
** Phase 2 - Checking Directories
** Phase 3 - Checking for Orphan Clusters
disk0s1: I/O error.
0 [Level 3][ReadUID 0] [Facility com.apple.system.fs] [ErrType IO] [ErrNo 5] [IOType Read] [PBlkNum 1744] [FSLogMsgID 1787532898] [FSLogMsgOrder First]
0 [Level 3][ReadUID 0] [Facility com.apple.system.fs] [DevNode devfs] [MountPt /dev] [Path /dev/disk0s1] [FSLogMsgID 1787532898] [FSLogMsgOrder Last]
disk0s1: I/O error.
0 [Level 3][ReadUID 0] [Facility com.apple.system.fs] [ErrType IO] [ErrNo 5] [IOType Read] [PBlkNum 1744] [FSLogMsgID 1585536958] [FSLogMsgOrder First]
0 [Level 3][ReadUID 0] [Facility com.apple.system.fs] [DevNode devfs] [MountPt /dev] [Path /dev/disk0s1] [FSLogMsgID 1585536958] [FSLogMsgOrder Last]
Unable to read FAT (Input/output error)
Free space in FSInfo block (364217) not correct (173941)
Fix? [yn] y
6 files, 86970 KiB free (173941 clusters)
So that's what I've found out for now.
What do you think I should do now? Should I try to erase and re-partition the disk to see if it solves the issue? Could these errors suggest the disk is about to fail?
I could technically keep using the Mac like this since the EFI partition isn't strictly needed to run Mac OS X and this problem only happens once at boot time (and I do keep a Time Machine backup). But should I?
Any thoughts?