The M6600 is constructed very similar to my M4600. As a matter of fact, they even use the same manual. The "Primary HDD" cage does indeed pop or "eject" out very easily by removing the battery to get to the latch.http://support.dell.com/support/edocs/systems/wsM6600/
You can review the product manual online and see how to remove the primary HDD. There is 1 single screw under the battery (remove the battery) that holds the "release latch" in place. Remove that 1 screw and put it in the original box for safe keeping. Then, all you have to do is:
1) Remove battery.
2) Slide the latch.
And the HDD just pops out the side. The HDD caddy uses no screws, so you can quickly swap HDD/SSDs. I use this for when I am travelling, and don't want my laptop subject to unwarranted searches. My internal mSATA PCI-E card in the laptop holds a bare-bone install with no browser history or files, in case they do search it.