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Replaced internal hard drive, won't mount, get I/O error. Why?

I followed the very good instructions from this site to replace the internal hard drive on my iMac Intel 20" 2105 with a 2 TB Western Digital SATA green drive. When I try to install Tiger 10.4.4, the new internal drive does not show up. If I go to disk utilities and try to format, erase, partition the new drive (which can be seen), I get an I/O error. I took the drive back out of the computer, put it in an external drive, formatted it, put it back in the computer, get the same problem. It shows up as a 14.6 TB unformatted drive that I cannot do anything with.

This is similar to a question raised in this previous thread:

Input/output error while formatting after replacing HD?

The answers there talked about Snow Leopard (which I don't think I can use with this iMac), as well as someone saying to jump pins 5 and 6 to slow down the drive (but no one ever said if they did this to successfully solve the problem). Also, the last question by Chip about his drive showing up as 7.3 gb and target mode not working was never answered.

Therefore, I am still at a loss as to what I should try. Can anybody help?

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Your systems specs: iMac 20" 2.0 GHz Core Duo


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I think you're hitting a few different problems here. Lets work though the different issues you are facing.

Lets look at the Western Digital Green HD spec sheet: WD Green series spec sheet. Note the SATA speed is SATA III (6.0 Gb's). Now click on the link above on your system specs. There you'll find your system is SATA I (1.5 Gb/s) sadly just on this one aspect this drive is too fast for your system without altering its SATA compatibility jumper. If your drive doesn't offer the jumper you'll need to choose a different drive that can be set to SATA I (1.5 Gb/s).

Now lets look at the next issue the drives size. This gets into what you used for setting up your partition: HFS or Extended (HFS Plus). If you have one partition which is Extended (HFS Plus) you'll be OK, If you setup your drive with HFS you'll need to redo it as it can't support your 2 TB drive in a signal partition. Here's a good Apple reference on OS-X's limits: Mac OS X: Mac OS Extended format (HFS Plus) volume and file limits.

I would encourage you to upgrade to at least Leopard (10.5.x). Your system can support upto Snow Leopard (10.6.x).

You should make sure your systems firmware is updated. Follow this Apple T/N: EFI and SMC firmware updates for Intel-based Macs.


We sorta have a chicken & egg problem here...

We need to have the newer EFI in the system so the drive I think will be the best fit for you will work correctly (keep reading).

First lets see if we can setup an external solution here as we'll need that. If I remember correctly I don't think it will boot up via a USB drive, I think it will only boot up via an external FireWire drive. If you want to give it a go, get a USB thumb drive (16 GB will do).

Using your recovery disk to boot up the system setup the USB drive (it is likely setup with FAT32 so you'll need to fix that) as a bootable drive then using the boot up manager (Option key) select it did it boot up? If not we then need to find a FireWire case to hold your HD and follow the below steps.

OK, on to a better HD drive:

The problem you are facing here is many of the current HD drives are now fixed SATA II or SATA III drives (even SATA II drives are getting thin).

What we need here is a drive that has auto SATA port sense so it automatically adjusts to the SATA port the system has. Seagate offers such a drive but unlike a standard HD it's a SSHD which also will give you some more zip! Here's the info on it: Seagate Desktop SSHD and here's the spec sheet: Desktop SSHD specsheet. Note the line SATA Transfer Rates Supported (Gb/s) is 6.0/3.0/1.5 So SATA I (1.5 Gb/s) is listed! Go ahead and install it into the external case so you can format it and install the OS on it using the same methods as the USB drive. If you got the USB drive working you can skip this step as we don't need the external case solution.

Now don't go crazy here! I would stay with the 2 TB drive or better yet, I would go with the 1 TB drive. While I understand your desire to have lots of storage there is a bit of give and take going on under the hood here. The bigger the HD the more RAM it needs to hold its tables. While its not much, you do have a system which is limited to 2 GB of RAM so you don't want to waste it on a large HD (or SSHD) table when you need it to run programs and data. (hopefully you have maxed out your systems memory).

OK, where are we now:

We either have a bootable USB drive or we have a external FireWire case to hold our new drive which has been formatted and prepped with the OS we want.

Now you'll need to down load the firmware upgrade (I assume you check it and its needed) and place the updater onto the bootable disk we have setup (USB or FireWire).

Once the firmware is updated you should be able to open your system up again and swap out the HD for the new SSHD drive and you should be working!

Let us know how it goes - Good Luck!

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Puan 2


Thanks Dan for your reply. In response to your suggestions, I tried jumping pins 5 and 6 on the drive, but that did not make the drive readable when it was put back into the iMac. Regarding the formatting and partitioning, I formatted as Mac OS Extended (Journaled), which I believe is the same as HFS Plus. So even with this, the 2 TB partition would not show up as readable.

I will try to get hold of a newer system disk to upgrade. Is it possible with my OS 10.4.4 install disk that I have been trying that the system is too old for the machine? I looked at a page listing the OS versions (, and 10.4.7 is the first listed as PPC/Intel. All previous ones were PPC only.


As for the firmware, I can't update it until I can get a system running on the machine. I only have installation CDs, no startup external disks. We have a TimeMachine backup of the drive that failed, but it can't be used as a startup disk externally (at least that is my understanding.

I have also put the old, smaller, original hard drive back into the iMac, kind of as a test to see if the computer is functioning properly and whether that drive would be recognized. It was recognized, and I could format it. So the computer seems ok. (I could not install the system on that drive for some reason….it said the disk could not be used as a startup disk and had a red exclamation point on the disk icon.)

I also ran a hardware test (restart while holding D key), both simple and extended, and both were fine.


So, I am down to 2 options. The new disk we bought is not compatible even with a jumper shunt on 5&6, and therefore we need to buy a slower drive. The system install disk I have might be too old for the machine.

Do you have any other ideas? Thanks.


Yea, some of the WD drives (newer?) OPT jumper appears not to work ;-{


Thanks again Dan. Just to be sure I understand your steps, here is a recap the way I understand you.

1--get a SSD with 1.5GB/s rate compatibility and put into an external USB case

2--put my TimeMachine backup/restore drive into an external Firewire case

3--boot with the backup drive

4--restore from the backup to the new SSD drive

5--download firmware updater to SSD drive

6--reboot from SSD drive

7--install firmware update

8--shut down and install the SSD drive internally and it should all work great

Did I miss anything or make a mistake?


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