how safe is an HDD encrypted with filevault? I have a macbook pro and I want to encrypt the whole system with filevault. 

My question is now, how secure are my data for removing hard disk from device and mount it in an external case?

For example someone steal my macbook, while it is in hibernate. Now, he won't shutdown the system clearly, open the case and remove the HDD.

Are the data already encrypted, even it is not shut down correctly?
Yes. FileVault 2 uses 128 bit AES encryption on all the data on your disk. This is why when you turn on FileVault, you will experience a background task grinding through your entire volume, encrypting the data as it goes. Once complete, only someone with your passphrase will be able to decrypt the data, even if they remove the drive from your Mac.