I believe my macbook pro hard drive is bad. I erased the drive and reinstalled the OS and the macbook still runs really slow. I dont know if the hard drive is the issue.

If you reproduce the problem with a clean installation of OS X, no other software installed and nothing restored from a backup, and with no peripherals connected, then it's caused by an internal hardware fault or by corrupt SMC data. To rule out the latter, see below.

Intel-based Macs: Resetting the System Management Controller (SMC) - Apple Support

It could be the hard drive, the drive cable, or it may need a reset. The following links may help you out:

Hard Drive Problems

How to reset your Mac's NVRAM, PRAM, and SMC | Macworld

Intel-based Macs: Resetting the System Management ...

How to Reset NVRAM on your Mac - Apple Support

MacBook Pro's are known to have some SATA cable issues, but I'm not sure which models they effect. A cable problem can be elusive because it can generate I/O errors in the log files just like a bad sector on a hard drive.

Very often when a new drive is installed the system won't recognize it until the system is reset. From the NVRAM reset link I posted above, here's a quote:

Information stored in NVRAM can include:

• Speaker volume
• Screen resolution
• Startup disk selection
• Recent kernel panic information, if any

As you can see, startup disk selection is in that list. I suspect the NVRAM is retaining information about the old drive but not the new drive.

Are you saying that you installed a new HD in your Mac, booted into Recovery

OS X: About OS X Recovery - Apple Support

and Disk Utility does not show the new HD?

What specific HD did you install?

Did you try an SMC reset too? It's easily done and none of those resets to the best of my knowledge hurt anything. When I was fooling with my system, which at one point was a home made Fusion setup, I needed to reset the SMC sometimes to get the drives recognized. Reading the documentation the sudden motion sensor is drive related but that seems to be about it.

Some more specifics may be needed about the drive itself, the model number of your unit, and the OS version you're running.

At present, the problem seems sort of odd to me. FWIW cable problems can be quite intermittent. I used Scannerz to find some of the problems on my original system that I later converted to a new HD and SSD combo. It can find lots of problems if you learn how to use it right (HD problems, logic board system faults, and cable problems) but you need to get a system at least partially up and running to use it, and it doesn't sound to me like your there right now (in other words, don't go out and buy it because at this point I don't know how it could help you)

Another word of advice would be to not go out and start throwing parts at a problem unless you have a good idea what the source of the problem is. If you have a logic board problem, new drives and new cables won't make it go away, you'll just be wasting your money. Logic board problems, cable problems, and drive problems can all have very similar end user symptoms. The article that I linked above named "Hard Drive Problems" has a section on general symptoms of problems and if you read the article it becomes clear there's a lot of overlap associated with different types of problems.

I assume that your unit isn't under warranty. If, by chance it is I wouldn't even fool with it and I'd take it to Apple and let them deal with the problem. You also might want to check with whatever Apple sources you have available to test your system, like an Apple store, and see how much they'll charge (if anything) to evaluate the system.

I certainly hope this helps.

Western Digital's site

http://www.wdc.com/en/

does not return anything when I search for

wdbmyh500anc

Maybe a bad drive. Can you test the drive in another computer or enclosure?

I think you left a zero out

http://www.wdc.com/en/products/products.aspx?id=800#Tab3

In any event it should work provided the HD is not defective.

Are you sure that the connector is fully connected?

Other than that the drive is defective since when you reconnect the old drive Disk Utility sees the old drive.

Could be that it was returned/open box because it did not work. There is nothing wrong at all with that specific model HD for your laptop.

I myself would get an SSD. They are very reasonably priced now.

All my computer less than 5 years old all have SSDs A lot faster

If you go the HD route try an Hitachi. They make a few 7200 RPM 2.5" models that are quite reasonably priced and quite fast for hard drives. I got one of them and a SanDisk SSD for my system. No problems at all, FWIW.

Good Luck.