iMac not recognising SSDs
I have a Mac Retina 5K 27" which is fully up-to-date. It has stopped recognising SSds. What should I do to diagnose the problem without returning to the Apple dealer at this stage?
Many thanks.
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Has it stopped regocnizing the internal SSD? Is the Mac still booting?
Assuming that this is for the internal SSD and the Mac is not booting:
Turn on your Mac, then immediately press and hold Command (⌘) and R until you see an Apple logo or a different image.
- ou may be asked to select a user you know the password for. Select the user, then click Next and enter their administrator password.
- From the utilities window in macOS Recovery, select Disk Utility and click Continue.
Choose View > Show All Devices (if available) from the menu bar or toolbar in Disk Utility.
The sidebar in Disk Utility should now show each available disk or other storage device, starting with your startup disk. And beneath each disk you should see any containers and volumes on that disk. Can't see your disk?
For each disk you're repairing, start by selecting the last volume on that disk, then click the First Aid button 
or tab.
In this example, the last volume on the disk is Macintosh HD - Data.
Click Run to start checking the selected volume for errors.
- If there is no Run button, click the Repair Disk button instead.
- If the button is dimmed and you can't click it, skip this step for the disk, container or volume you selected.
- If you're asked for a password to unlock the disk, enter your administrator password.
After Disk Utility has finished checking the volume, select the next item above it in the sidebar, then run First Aid again. Keep moving up the list, running First Aid for each volume on the disk, then each container on the disk, then finally the disk itself.
If Disk Utility can't see your disk, it also can't see any containers or volumes on that disk. In this case, follow these steps:
- Shut down your Mac, then unplug all non-essential devices from your Mac.
- If you're repairing an external drive, make sure it's connected directly to your Mac using a cable that you know is working correctly. Then turn the drive off and back on again.
- If your disk still doesn't appear in Disk Utility, your Mac may need servicing. If you need help, please contact Apple Support.
If this is not about your internal SSD and your Mac is still booting: run disk utility via launchpad>type disk utility and hit enter. Then follow the steps above to the disk that is not working