I've got a 700GB external disk. Can i safely turn on bitlocker without erasing any data?
Drive encryption programs do not erase the data on volumes they have been turned on on. They will encrypt all the data, which can take some time depending on the type of encryption, the amount of data to be encrypted, the speed of the drive and system, and many other factors. But unless there is a catastrophic failure during the encryption process, your data will not be deleted during this process.
That said, if an encryption process fails catastrophically (I have not yet observed such an occurrence myself, the chances are VERY remote), you're less likely to be able to recover the data than if an unencrypted drive experiences a similar problem. This is simply the nature of encryption. But the chances of this are extremely remote and so should not generally be considered a reason not to encrypt.