[lug] sfdisk safe on running system?

[stimits at comcast.net]

Hi,
 
I have two drives in software RAID-1 (sdc and sdd) with LVM2 on top of that (my "/home" and some other non-rootfs directories use the LVM on this...the main Fedora 27 o/s is on its own sdb drive, Windows 7 is on sda). At some unknown event the mirror sdd drive ended up being disconnected from the RAID-1 array and is no longer participating. I think this was from a short time back when I had to re-install Windows 7 because now Windows 7 sees this and says it is NTFS (complete with a Windows UUID). So I want to put this back in the array.
 
Is it safe, on a running system to set up this unused/unmounted sdd via sfdisk to copy from sdc structure while the system runs (don't know if this matters, but the disks are all old style BIOS, not GPT)? Example:
sfdisk -d /dev/sdc | sfdisk /dev/sdd
 
I haven't found any information on this which I trust. The other question would be that if I can do this safely, is it safe to merge the sdd back in while the system runs? I would think it could do this, although might be degraded performance until done. Example (this is "/dev/md127" in this case, and the first partition occupies almost the entire disk):
mdadm --manage /dev/md127 --add /dev/sdd1
 
Any thoughts on whether this is safe to do? Or perhaps I'm better off with the LVM partitions on top of this being umounted?
 
Thanks!
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