Mid 2009 MacBook Pro HDD/SSD upgrade suggestion?
I upgraded this model to a 480gb SSD but I had to do two things. First, I replaced the disk drive cable with a new cable for 2010 MBP 15" from iFixit. Second, per OWC, I used an SATA2 rather than an SATA3 drive.

Testing showed the cable replacement was absolutely required. I didn't have an SATA3 drive to test against so I'm taking that one on faith.

Assuming your replacement drive is an SATA2 drive, you just need to replace the cable.  It's not hard but e careful with the screws that attach the cable to the body. I stripped one and it was a PITA.

It's a whole new machine with an SSD. Very fast for everything except CPU-intensive stuff like encoding video.