Hey Sandi.
Sorry to hear about the problems you're having with your drive. :\
Having in mind you're getting better readings while you have the drive connected externally might mean that the SATA port and/or cables might be at fault. Give it a go with a different SATA port and cables if possible, or even better - with a different computer, to see if the same thing happens.
It is advisable that you backup your data first, though.
After that go ahead and test the drive by running both DLG (Data Lifeguard) tests (Quick and Extended), to see if anything alarming pops-up:
How to test a drive for problems using Data Lifeguard Diagnostics for Windows Once this is done, if there's nothing out of the ordinary, you could follow @SkyNetRising's advice and double-check the drive with a 3rd party tool as well, if you want to.
Hope that helps. Please keep us updated.
Boogieman_WD