It also depends on how you import pictures to lightroom:
When you import images to Lightroom, Lightroom gives you four different ways to do this, and they're all at the top of the Import window. You select which one you want by clicking on it. From left to right, the options are:
Copy as DNG. This converts your RAW images to DNG (Adobe's digital negative) file format as the files are imported. The RAW files are left on the card. This has the tradeoff of not requiring sidecar .xmp files to store the edit history of the file, but can lose you proprietary metadata of the specific RAW format you're using.
Copy. This copies the photo files from where they are into wherever you tell Lightroom to put them as you import. This will leave all the files on the card alone, which can be useful if there's a problem with the import, you want to keep the card for backup, etc.
Move. This moves the photos from where they are into wherever you tell Lightroom to put them as you import, which will automatically remove the files from the card.
Add. This just adds the photos to the Lightroom catalog where they're resident. This can be handy if you like to copy the files from the card to where you want them to live on your OSX box by hand before doing the import.