Ubuntu: Automount /etc/fstab with mount -a, does it disrupt already mounted devices?

Say I have /dev/sdb and /dev/sdc already mounted and I am currently copying files from one to the other. Can I safely edit /etc/fstab to also include yet unmounted /dev/sdd and do mount -a to mount it? Or is there a risk that a process working on the other already mounted drives will be disrupted? (I.e. will those be unmounted and remounted in the process, or is mount -a clever enough to just ignore them?)

The mount -a command will detect already mounted filesystems, according to its man page. So running it after editing /etc/fstab shouldn't do any harm to the already mounted partitions.