Hard drive not mounting?

Hey, trying to trouble shoot a Macbook Pro startup problem:

mid-2010 15" MBP

couple of days ago booted up and had grey screen with question mark folder.

tried resetting MSC and NVRAM, to no avail.

further reboots had a apple logo crossed out or just grey screen and nothing.

no chimes.

-Pulled the drive, which was a stock seagate, placed in external case and came up as not recognized.

-booted from external, worked fine.

-assumed this meant a good ole drive failure, bought a new SSD, popped in the MBP and while booted from an external, tried formatting drive, and it wouldn't format.

-took SSD back out, formatted and installed OSX while it was in an external case, booted while connected to external case.

-put SSD back in MBP, get nothing but a grey screen.

given that the orginal HDD wouldn't mount in an external case i assumed that just meant a bad drive, but clearly something else is up? Bad SATA cable? it seems curious that the original HDD is no longer readable AND now the new one won't boot? is this a logic board issue?

any thoughts?

THANKS!