Macbook air 2015 unresponsive

I'm a bit confused on what "SSD" you removed since you mentioned connecting a hard drive.

If connecting an external drive causes a problem, then either the file system on the external drive has an issue or the external physical drive has a hardware issue, or maybe a cable issue. Perhaps the port on the laptop has a problem, or the external drive has a compatibility issue with other connected devices.

You can try running Disk Utility First Aid on the external drive to see if there are any issues with the file system. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. Any unfixed errors means you will need to erase the drive and restore from a backup assuming the physical drive is still healthy.

You can try booting into Safe Mode to see if it works properly...assuming you are not using the NTFS file system on the external drive. Safe Mode prevents third party software from automatically launching during boot & login. Typical third party apps which are known to cause problems are anti-virus apps, cleaning/optimizer apps, and third party security software....none of which are needed on a Mac. If you are using these types of apps, then uninstall them by following the developer's instructions.

Try connecting the drive to another computer to see if the problem follows the drive.

Yes, I find that is very common on the 2012-2017 MBAirs since the cable & connector for the Trackpad are beneath the opening at the top of the Trackpad.