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Apple service provider in your area, or any reputable shop you feel comfortable going too.
 
What year is your iMac?
If you have USB, just add an EXTERNAL USB3 SSD and set it up to be the boot drive.

The Mac will run MUCH faster, and you'll never even notice that you're "booted externally".

You DON'T need to pay anyone to do this.
You can do it yourself, easily.
 
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