RAID1 with only ONE member?

[Andy <andy@besy.co.uk>]

Hello List,

I am building a new backup box and would like to use linux software
RAID-1. But I can only afford to buy one disk right now.

Can I configure a RAID-1 array with only a single member for the time
being, and then add additional member disks at a future date?

Is is simple and safe to add additional members? I have only used the 
Debian installer to configure the software RAID thus far...

Thanks, Andy