Sometimes when you clone an OS X installation, for example using Carbon Copy Cloner, it will create a Recovery HD partition but not hide it for you.
The “hidden” status of a partition is determined by it’s partition type – in the case of the Recovery HD partition, it’s set to type Apple_Boot and this hides it from Disk Utility and prevents the Finder from mounting it.
If you have one visible and want to make it hidden, then there are a few quick commands you can run in the terminal to fix it’s type;
First, find what device contains the Recovery HD that’s mounted:
mount | grep "Recovery HD"
This will return something like the following where I’ve hilighted the important bits in bold.
/dev/disk2s3 on /Volumes/Recovery HD (hfs, local, journaled)
We can see that the Recovery HD is /dev/disk2s3 in this example (it will probably be different for you, so don’t take my word for it!)
Next, run the following two commands, replacing /dev/disk2s3 with whatever you had returned above. The first command unmounts it from the desktop so we can have greater control over the device, the second one adjusts it’s partition type to an Apple_Boot partition so it stays hidden as it should be.
/usr/sbin/diskutil unmount /dev/disk2s3 /usr/sbin/asr adjust --target /dev/disk2s3 -settype Apple_Boot