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Please which SSD blade is the best one, fast and functional for my iMac 27 late 2012 Thank’s

There are three different sized blades made by OWC listed on this page for your machine, your choice may be about how deep your pockets are: https://eshop.macsales.com/upgrades/imac

If you are talking about the PCIe blade SSD Apple uses, you have no choices as Apple uses a custom design. Apple does not offer it as a replacement part only as an exchange, so you’ll need to find one from a parts system. You might want to think this through as opening the newer ‘Thin series’ systems are a bear!If you don’t use the proper tools and techniques you can damage the display which would be an expensive Opps! Here is the IFIXIT guide you’ll need to follow: iMac Intel 27" EMC 2546 SSD Replacement If you currently have a Fusion Drive config you’ll need to first break the fusion drive set first, following this guide: How to split up a Fusion Drive