Defective cable for internal hard drive
Totally easy! Took less than 10 minutes to install cable. Apple Store refused to install cable because tech noticed some sticky reside on logic board. Very minor and I removed the reside myself. Installed the cable, and now my mid-2012 Macbook Pro has new life again. 5 years going and strong!
You can do it! Buy the part. Take your time. Install it. You can fit it!