Macbook boots up to grey 'prohibited' symbol I'm having an issue with my '09 13" Macbook Pro. When I boot it up, it seems to be booting normally until about thirty seconds in, when the Apple symbol is replaced with a kind of 'no-smoking' symbol, without the cigarette in the middle.

I've removed the drive one and put it in an external holder, and repaired it using Disk Utility on my Macbook Air. It found some things, and said it repaired them. I then put the drive into my Macbook Pro again, and it booted once to only one of my partitions. It acted very laggy and slow, and eventually I had to force kill it with the power button. Now it does the same thing.

I'm in the process of removing the hard drive to try to repair it again, but I wanted to know if anyone here has any ideas on what this could be. Is it the machine itself somehow?

I see this:
There are times when Apple's 'Disk Utility' software isn't as thorough at repairing problems that other applications may be.
I've had 'Disk Utility' tell me that drives are fine, but it is not until after running a program like 'Disk Warrior' (that replaces the Directory structure), that the hard drive feels 'normal' again.
This isn't to plug any particular software, but to let you know that 'Disk Utility' isn't the be all, end all of determining if a hard drive is 'good'.

Given your description of the problem, I'd be inclined to make sure I had a backup. If not, purchase a new hard drive, and get all the data off of the old one as quick as possible. (I agree with @Asmus that it appears that you are about to lose a hard drive).

On the other hand, you may just need to have the mac reinstall the OS, and fix things that way. Either way - make sure you have a solid backup right away.

Good luck.