Back ups

[yura]

I just bought a new an external hard drive I am using as a back up, it is connected with my MacBook Pro through Time Machine.
Is it OK to leave the external hard drive on all the time even when the computer is off?
When it back up after the first time, does it back only the new findings or backs up everything again.
Because I have very sensitive information, I would like not to loose it, can I use two external hard drives?

 

[Kaveman]

Is it OK to leave the external hard drive on all the time even when the computer is off?

Sure, mine are on 24/7, but over time the enclosure fans do wear out and can be noisy.

When it back up after the first time, does it back only the new findings or backs up everything again.

Just the changes you have made.

can I use two external hard drives?

ATM TimeMachine only backs up to a single drive, but there is nothing stopping you from doing a snapshot to another drive overnight every day. I use Carbon Copy Cloner to do this also.