You have the right idea of monitoring Activity Monitor. Not sure in its self it will help you here.
Have you tried removing the newer RAM modules and/or put the old ones back in?
What is the make and model of your new HD? This systems HD SATA port is only SATA II (3.0 Gb/s) make sure your drive is able to run at that SATA speed.
I would also double check your systems firmware to see if its up to date. Here's the Apple TN: [http://support.apple.com/kb/HT1237|EFI and SMC firmware updates for Intel-based Macs]. Only problem here is if you try to update it and the system crashes in the process you'll trash your logic board so I would hold off changing it. What you want to see is if it's the latest for the system or not.
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== Update ==
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It appears you have put in a slower drive than what the system can handle. The Fujitsu drive is a SATA I (1.5 Gb/s), your system is designed to support a SATA II (3.0 Gb/s) HD.
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Here's the specs for your system: [http://www.everymac.com/systems/apple/imac/specs/imac-core-2-duo-2.4-20-inch-aluminum-specs.html|Apple iMac "Core 2 Duo" 2.4 20-Inch (Al)]
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I would try a SATA II (3.0 Gb/s) drive and see if your system stops freezing.
You have the right idea of monitoring Activity Monitor. Not sure in its self it will help you here.
Have you tried removing the newer RAM modules and/or put the old ones back in?
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What is the make and model of your new HD?
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What is the make and model of your new HD? This systems HD SATA port is only SATA II (3.0 Gb/s) make sure your drive is able to run at that SATA speed.
+
+
I would also double check your systems firmware to see if its up to date. Here's the Apple TN: [http://support.apple.com/kb/HT1237|EFI and SMC firmware updates for Intel-based Macs]. Only problem here is if you try to update it and the system crashes in the process you'll trash your logic board so I would hold off changing it. What you want to see is if it's the latest for the system or not.
You have the right idea of monitoring Activity Monitor. Not sure in its self it will help you here.
Have you tried removing the newer RAM modules and/or put the old ones back in?
What is the make and model of your new HD?