To just unmount but preserve the filesystem go to Configuration -> Storage -> right click on the storage and select unmount.
Now you can hot swap the drive if it's supported by your hardware or you have to shut the host down and remove the drive.
edit: You have to remove any VMs residing on that storage from inventory before doing that (Not delete from disk).
I don't know how to restore the vmfs filesystem but I would recommend to make a DD image of the drive immediatly and keep it at a safe place before doing anything else.
Regards,
Benjamin