iMac 20" lags, probably HD failing, need help identifying the cause.

Hello everyone,

First of all I am posting this thread here because I am pretty sure that I have a hardware problem. I run iMac 20" 2.4 GHz 2 GB RAM 250 GB HDD WDC WD2500AAJS-40VWA1, 128 Mb ATI Radeon HD 2400 XT.

A week ago I started having little lags while playing world of warcraft, there were other programs like skype, mail, firefox, transmission running also. I didn't pay much attention to it because I thought it was because of the transmission, which always lags up the system while running. Then this problem started to increase. The lags became longer and appeared more often and not only on the WOW anymore, but doing basic daily computing. any programs like firefox or skype which are running become unresponsive for some time when i try to do anything with them, then after a while become functioning again but for a short period of time.

Then I tried to identify the problem and carried out these steps:

Repaired the permissions and disk with DiskUtility booted from SL install DVD. No problems were found, but the lags persisted. Eventually my system froze completely - couldn't boot up the OS X. So I booted the Win XP on separate partition and backed up the important files.

I erased and installed a fresh copy of SL, updated it via Software update to 10.6.3. But the permissions went totally crazy and there were countless number of them. Then I updated OS X manually with Combo update from apple support site. The permissions were all fixed. I used the system as I did before and the lagging problem reappeared.

I ran all the tests with Onyx and it could not find anything wrong, the same as Disk Utility and the same as Tech tool pro running right now, although the HD sector scanner has not yet finished.

I thought that it is my HD that is causing this problem, but the win xp ran without any glitches or anything.

I've tried reseting the PRAM, did not help. Then I have merged the two partitions into one, used the free space eraser with zeros and installed again fresh SL but i did not update it, just installed skype and utorrent to download some tools. The lagging is still constant for a longer or shorter period of time, even when Im typing this post.

i have been checking the disk activity in activity monitor and HD shows really big fluctuations at times. While running skype, firefox and tech tool pro it averages around 0.5 kb to 1 mb peaking. But sometimes when the lag occurs and programs become unresponsive the data read/write shows 0 bytes and suddenly peaks up to 40 or 60 Mb/sec.

To sum up:

The problem keeps increasing the system becomes totally unusable. The software problem was ruled out due to continuous and various steps taken mentioned above (SL erase and install tried 3 times).

Is it the sign of HDD failing? Please help me determine the cause of the problem.

Firstly, welcome to Mac-Forums; and congratulations on a thorough report of the problem.

Lag is often a sign of a logic board malfunction rather than OS or hard drive.

Your iMac is about two years old, judging by the specs you've given, which suggests that it's out of warranty. If you have ongoing AppleCare, you may get the logic board tested, and replaced if necessary, at minimum cost.

In any event, I'd recommend that you take your iMac to an Apple technician for thorough hardware analysis.

Thank you hughvane for the reply.
You are absolutely right, my iMac is manufactured in September 2008, Im out of warranty.

An update:

I have just installed the WD Caviar Black WD1001FALS 1 TB - SATA-300 32 Mb cache. I tried to be as careful as possible because it was my first iMac "opening" ever. Hopefully everything went smoothly and the SL has just been installed.

From the first sight, the system runs noticeably smoother, without any interruption of lag, beach ball or any kind of freeze. I checked the activity monitor instantly while downloading skype, vlc and firefox dmg files and the peaking per second and both read ins/outs numbers were significantly improved.

The conclusion so far is that the previous HD was near to its end. I will run techtool pro later to check the system. If it finds anything abnormal I will post it.

P.S. When I opened my mac I found loads of dust inside, so I would suggest to perform a dust cleaning every once in a while. Just to prevent the overheating of any boards or other devices.

P.S. When I opened my mac I found loads of dust inside, so I would suggest to perform a dust cleaning every once in a while. Just to prevent the overheating of any boards or other devices.

Click to expand...

Always a good idea, however, opening an iMac is not something to be taken lightly. (as you probably found out) I suggest carefully looking over one of the many videos that are on the web which give instructions on how to do it before actually attempting to disassemble the machine.

Regards.