Prohibitory symbol after fresh install on new SSD

The symbol you see means the computer hasn’t found a bootable system drive and sometimes occurs when you haven’t set a default startup disk. Go to System Preferences and select Startup Disk. You should see your SSD in the window. Select it after you unlock the preferences.

The odd thing is that if this is your problem it should resolve itself in a matter of seconds - certainly not hours. This suggests there is something more involved. You say you’ve tried replacing the SSD 3 times. Different drives? Different manufacturers?

Hi MrSSDGd:

Here are a few thing to try:

Under System Preferences > Startup Disk, ensure it is pointing to your new drive.

Sometimes if it is not, it may eventually find it, but it may take a while.

1. Set the default startup disk.

2. Reset the System Management Controller (SMC) on your Mac - Apple Support

Reset SMC:

Shut down the Mac.

Plug in the MagSafe or USB-C power adapter to a power source and to your Mac.

Using the built-in keyboard, press Shift-Control-Option on the left side of the keyboard, then press the power button at the same time.

Release all keys, then press the power button again to turn on your Mac.

3. How to Reset NVRAM on your Mac - Apple Support. Afterwards you’ll need to repeat Step 1.

Kim

Take the SSD out and install it in an enclosure, connect it to the MBP via USB and boot the MBP. If there are no problems, it points to a faulty internal SATA cable. Replace it.

Ciao.

What does DiskUtility say?

It should be partitioned/formatted as GUID Partition Table, OS X Extended (journaled).

Open Terminal (in Utilities) and copy paste this command

diskutil list

then post here what it displays.

As OgelThorpe has indicated, your SATA cable may also be the issue.

MacBook Pro hard drive SATA cable issue. MacBook Pro Hard Drive Cable Failure-Free fix and preventative maintenance

If your SATA cable is faulty, here is a guide on how to replace it.

MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement: iFixit.com

MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable: SATA Cable

Kim