They might swap the drive from the current machine to the new one (I assume they have agreed to a machine swap?) if you ask them very nicely.
This would be the easiest solution if the Apple store will do it.
Best option. buy a new HDD online, and then swap out the new one with your current one.
Take it back, then put your HDD in. sell the HDD it came with for a profit. everyones happy.
This would necessitate opening up the current iMac and then the replacement iMac. Might not be the easiest thing to do if you are not comfortable doing this type of thing. YMMV.
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I would suggest this plan of action:
1. Purchase an external HDD that is the same size or bigger than your current internal HDD. Note, bigger is always better and allows you to do backups later on.
2. Using Carbon Copy Cloner, clone you Mac partition. You will need to format the external HDD using the GUID partition scheme. Once done, make sure that you can boot from your external HDD. Assuming that you can, proceed with step 3.
3. Use WinClone to clone the Windows partition. It will create an image. Store the image on your external HDD and not the internal HDD.
4. Boot to the external HDD and then using Disk Utilities do a secure erase of your Internal HDD.
5. Shut down the iMac.
6. Remove the external HDD.
7. Boot your iMac using the restore DVD that came with your iMac. Install (restore) the system.
When you receive your replacement iMac, to the process in reverse.
1. Connect your external HDD and boot from it.
2. Erase your internal HDD.
3. Using Carbon Copy Cloner clone from the external to the internal HDD.
4. Using WinClone, restore your Bootcamp partition.
Note, if you store your WinClone image, which will be a little bigger than your Windows partition, on your external HDD, and there isn't enough space remaining on your internal HDD for the partition, you may nee to delete the image from your internal HDD (it will still be on your external HDD), and then use the WinClone to restore from the image on the external HDD.
Note, it's late here, so if you have any questions, please respond here and I will check tomorrow.
EDIT:
Regarding WinClone:
- I've had good luck with Windows XP (32 bit).
- Others I know have tried it with Windows 7 and it worked okay. Not sure if it was 32 or 64 bit versions.
- As of July 2010, it looks like it has been discontinued which is sad to see. The latest version is 2.2 which you can still download.
YMMV.