My Macbook Pro 2012 intermittently freezes/lags, except when booted as an external drive

stick bugged gif Said:

"My Macbook Pro 2012 intermittently freezes/lags, except when booted as an external drive:"

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Q1. "If you believe that a computer should only be fast for 3-4 years then you are being duped by the major companies who keep you on their treadmill of consumption.[...]"

A1. Apple allows for 7-year updates and then no mare allows for update to be added. Don't get me wrong, I have older Macs, just for troubleshooting purposes. Oldest I have at the moment is a 2000 iMac. With items no longer having the option for macOS update installations, you are ore more prone to failure bug installations.

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Q2. "own a 2012 MacBook Pro for a specific reason. It is newest reparable Macbook, as Apple doesn't do consumer-repairable anymore. It is ample for my usage, and this issue happen suddenly and in an extreme way. It is unrelated to your notion that 'slowness catches on' because I have 16GB of RAM and an SSD."

A2. Seems as if something is causing your Mac to lag, after login. So, perform the following...

Troubleshooting Lagging Mac:

A. Try Safe Mode:

In Safe Mode, login items do not run, and corrupt system files(i.e. caches) are intentionally reset as a way to fix issues

1. Boot: in to Safe Mode: [Hold Down: shift key upon boot].
2. Log in: as same user
3. Wait: 30 seconds
4. Restart; your Mac
5. Log in: as same user

B. See if your Hard drive is Failing:

What to Try:

• Verify your Drive: Repair your Disk in Disk Utility - Apple Support
• Go Here: If your Mac Starts Up to an Apple Logo or Progress Bar - Apple Support
• Perform the following in Recovery Mode:

1. Boot: into Recovery Mode. Go Here:  How to Use Safe Mode on your Mac- Apple Support
2. Select: Disk Utility
3. Select: the Drive
4. Click: First Aid button

C. Remove Login Items:

While in Safe Mode, remove all of your Login Items, and then restart the Mac.  Go Here: Change Users & Groups Login Items Preferences on Mac - Apple Support

Great troubleshooting. Most likely the internal hard drive SATA Cable is at fault if the SSD works fine when booted externally. The only other things it may indicate are a compatibility issue with that SSD and the laptop or a bad Logic Board. Not all SSDs are compatible with all computers when used internally. I'm just putting this out there to be complete as I am fairly certain the SATA Cable will solve the problem.

You should uninstall Sophos by following the developer's instructions. Anti-virus apps, cleaning/optimizer apps, and third party security software are not needed on a Mac and usually causes more problems than they solve plus they impact system performance. macOS has great built-in security especially when combined with the user practicing safe computing habits as outline in this excellent article written by a respected forum contributor:

Effective defenses against malware and other threats - Apple Community

stick bugged gif Said:

"My Macbook Pro 2012 intermittently freezes/lags, except when booted as an external drive: [...]Any other suggests for how to troubleshoot this further, given that the MacOS works just peachy when booted to as an external drive?"

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It's Time to Upgrade:

Your computer is almost more than a decade old. Computers typically go fast for 3-4 years, then slowness catches on. So, take this as a good reason to upgrade. And that's what I have to say.

It looks as if you have taken main steps to identify issue. I would be interested to find out whether the replacement SATA cable solves your problem - perhaps you could update when complete? Good luck

Thanks for update. Fingers crossed that you are back up and running for foreseeable future!