I have a lacie external hard drive, and the encryption has not finished. Will it still be able to restore back up if I do not allow it to finish?

It is always advisable to allow the process a reasonable amount of time to finish

If you interrupt - you do run the risk of messing up the formatting and structure of the drive

Interrupting a full disk encryption is one of the most reliable ways of causing a full disk failure and total content loss.

On the bright side, all the above is software caused, so you'll have an empty drive to erase and fill afterwards!

You can interrupt it. Not a problem at all.