Not sure if this should be a HW or SW question, but basically having a hard time figuring out if I can have a drive up and running and then after the fact buy an identical drive and mirror it into a RAID1. Searching brings up yes, and no. I know how RAID works but I've never done it so not sure where to go. Just trying to be cheap for a couple weeks in between drive purchases.

It depends on how the filesystem is.

btrfs will let you do that. As far as I know, none of the others will.

md will NOT do it UNLESS the partition is first initialized as a raid 0 with only one disk. The initialization of the partition as an md device will occupy some of the blocks that would otherwise be used as the superblock. Once initialized as a raid 0 I believe you can then modify it to a raid 1 in degraded mode, then add a second disk. The action of adding the second disk will then replicate the configuration, and copy the contents.

This is because md raids are emulating a raid controller device, and the raid handling is separate from the filesystem.

ah. My post above is all about software raid, not hardware raid.

A hardware raid controller would have its own handling. As to whether it would allow it or not, depends on the controller, and how the disks are handled. Most require some blocks to be used for the raid configuration, and if the controller is not being used in a raid configuration, then these blocks are not initialized, and you are back with the same limitations as with md raid.

Yes - I'm surprised your searching didn't find this.
Stick the new disk in, partition as appropriate (or not at all), create RAID1 with 2 devices and --missing using the new disk/partition.
Maybe have to reboot to get the new disk/partition(s) recognised. Check /proc/mdstat that everything is ok, mount it, mkfs, copy/rsync.

At this point, for a data disk, you can do a reboot and check it looks ok, then just trash the old disk, re-partition it and add it to the RAID. Simple - it will sync itself across. Fix fstab in need.

If it's your system/boot partitions we are talking about you need to be a little lot more careful - bootloaders and initrd spring immediately to mind. And of course that is distro specific.