Based on my experiences with Megasync for Mac 1.0.39:
- When you disconnect the drive: Megasync will stop syncing folders that were on that drive. No files are removed from the local machine or from the cloud.
- When the drive is connected again: Megasync will not enable syncing automatically. You'll have to manually enable syncing of folders on the external disk.
One annoyance I discovered was that Megasync might lock your external drive and prevent ejecting of that drive. In this case a manual disabling of sync is not needed.
I did not find any documentation regarding this matter from Mega website. Therefore I cannot confirm that my experiences apply to other users.