MacOS – MacBook Pro won’t boot with new hard drive, but a different MacBook Pro will

So here are the details. My friend and I both have 13-inch Mid 2010 MacBook Pro's. Same processor and specs since they were purchased at the same time, but originally had 250 GB hard drives. His hard drive failed and needed a new one. Put a 500 GB Seagate Momentus 5400.6 in his computer, and tried using a flash drive with "Install OS X Mavericks" on it to install Mavericks on his new hard drive. No matter what, it wouldn't install. I finally was able to install OS X 10.9.4 on it by connecting the new hard drive to my computer with a USB hard drive enclosure. I put the hard drive back inside his computer and it wouldn't boot (stuck on the gray screen with apple logo and a spinning wheel). I then attached to the hard drive to his computer externally with the USB enclosure, and then it booted and worked perfectly.

So, it seemed like the problem was with something inside his computer, but then I switched our drives, and that doesn't seem to be the case anymore. I put my original hard drive into his computer, and his new hard drive into my computer, and they both booted and worked perfectly, so I have no idea why his computer will not boot with the new hard drive internally.

To summarize, his computer boots with: 1) new hard drive connected externally (won't work if it's internal), and 2) my hard drive internally
but my computer boots with: 1) new hard drive connected externally AND internally, and 2) my hard drive internally.
Both hard drives have OS X 10.9.4 installed

If anyone has any idea what's causing this and how to make his computer work with the new hard drive internally that would be greatly appreciated.

Edit: Update
I was able to test the computers with some other hard drives. I have his original hard drive (which apparently needed to be replaced), which I was able to read with my computer and install Mavericks on. I also installed Mavericks on a 1 TB Toshiba hard drive. Here's the results with all 4 hard drives installed internally on either computer:

His computer:
1) Works perfectly with the original hard drive from my computer (recall they are the same model MacBook Pro)
2) Stuck on gray screen with apple logo and spinning wheel when booting with 500 GB Seagate and 1 TB Toshiba hard drives
3) Folder with question mark when booting with his original hard drive (which is why he said/was told it needed to be replaced originally)

My Computer:
1) Works with all 4 hard drives, including the one that causes his computer to show a folder with a question mark.