Can't Login

Macbook:

I turn on.

it does its thing.

Grey screen, then to blue screen which typically (i think) happens right before it loads the login screen, but instead I get a spinning wheel, page sort of refreshes a new wheel and continues spinning, cycle continues.

I'm running snow leopard.

Background:

I recently cloned my original HDD which is strictly SL to an external HHD. I partitioned the external HDD in two parts. The clone has booted fine for two days. After I cloned to the external HDD, I switched the drive putting it in my macbook, ran boot camp and installed Windows XP pro. Ran Fine. Installed Office Enterprise...worked fine.

I used this set-up a few times logging in and out, etc. No problems except that the time was a bit off after using windows. WTF. I really don't want to have to re-clone SL. Or I don't want to run SL install all over again and have time machine do its thing, because I don't want to have to deal with problems of certain applications not working properly, since I heard that this can cause some issues with transferring serials e.g. CS4.

ERRR Please some advice much appreciated.

cheers.

Oh, and when I hold option, I get the option screen that lets me choose which OS to boot. That works fine, though I did not try to boot windows.

 

Thanks you for commenting:

I swapped the drive out. If I recall correctly it's possible to boot OSX from external HDD. What about safe boot from External HDD?