Will Time Machine backup a RAID 1 Drive

Short: one set of files is backed up.

Long: you can think of the storage system as layered. At the bottom is a layer of disk partitions representing disks; see /dev - as in ls -l /dev/disk or file /dev/disk

At the next layer up, these partitions can be used directly (no RAID) or grouped using RAID. At the next layer up, the (non-)RAIDed partitions can be used for a filesystem; see /Volumes - as in ls -l /Volumes

You can see the mapping of disks and filesystems using the mount command (no arguments - repeat - supply no arguments to the command): mount

Time Machine works at the filesystem level and backs up files and directories; Time Machine does not see the (non-)RAID or partition levels. Another way to think about it - Time Machine sees file names and attributes (read, write, owner, group, etc.), and those attributes and names do not exist at the RAID level.

Time Machine backs up on file system level, so it doesn’t matter whether your file system is on a simple volume or a RAID configuration, all it backups are files and folders. This also means that you can restore a TM backup to a RAID configuration if required.