Should I have my Mac on until "external" hard drive encryption process is done? I have a MBP 2012 and replaced the internal hard drive with an SSD and the SuperDrive with a 1TB HDD. I'm encrypting my 1TB HDD. 

Should I leave my Mac on until it's done? 

Does OS X pause the encryption process if I turn my Mac off (or put to sleep) or does it start over every time I turn it back on?

I'm running El Capitan 10.11.5.
Not necessarily. 

OS X seems to handle well pausing and restarting where the encrypting process left off. 

However, there is no way I can recommend doing this. I had backups of my data and decided to try it. :) I just wish there was more official information regarding this process.