Installed new SSD, won't boot, can't unmount

I got a new SSD from Crucial to replace the HDD that Apple said was dead.

Old HDD would shut off during boot, and said it was full in recovery mode (wa only about half full).

Got the new SSD, erased and setup as GUID partition, installed macOS.

I installed the SSD, and it would hang half way into starting up.

I made an external hard drive a bootable drive and booted from that.

When booting from external drive, I can't run first aid on the internal SSD because it says the disk can't unmount because it's in use.

Tried running "lsof -l +L -R -V | grep /Volumes" in terminal, and nothing showed up.

Is there something else I need to do to set the SSD up right?

I saw one post saying the HD cable could be the cause.

Thoughts?