well, i guess it could look like a single volume, somewhat...
first, im going to assume this is a native setup, so there is plenty of room on your main hard drive. so, you have "/dev/hda" mounted to "/". this is your primary hard drive that has linux installed on it. then you have your secondary hard drive, that is going to be used for storage. this one is "/dev/hdb" and it is mounted on "/mnt/hdb1" (1 means first partition).
then, you configure samba to share "/media", or whatever. then make a symbolic link to "/mnt/hdb1" inside the '/media" directory ("ln -s /mnt/hdb /media/hdd2"). that way, when you look at the share from another computer, you will see the contents of your primary hard drive's "/media" directory and also see the contents of your other hard drive in the "/media/hdd2" directory.
of course this isnt really like raid, each folder would be independent of o f eachother, but you could see them both in one share. if you really need raid, it probably could be done, however i have never setup a raid array, so i would not know where to begin. im sure some googling and trial and error and you could do it.
ps: just a quick afterthought on my idea. say your primary hdd is a bit smaller than your other one, so in "/media" you have a directory "music". then you have your large hdd symlink, but it is named "videos". so your smaller hard drive will store music files, which use less space, while your larger hard drive will store the space eating videos. of course this same idea could be reversed (larger hard drive has linux filesystem and music, with smaller hard drive symlinked to "/media/music".) hope i could help.
This post has been edited by bc54: Jul 22 2009, 06:08 AM