Based on the documentation I've read, I'd agree with eirkatcuse...the minimum configuration is two drives.
From the "device setup" webpage (http://www.lime-technology.com/wordpress/?page_id=31):
"An unRAID server disk array consists of a single parity disk and a number of data disks. The data disks are exclusively used to store user data, and the parity disk provides the redundancy necessary to recover from any singe disk failure."
and later...
"Unlike traditional RAID systems which stripe data across all the hard drives, an unRAID system stores files on individual data disks. Consequently, all file write operations will involve both the data disk the file is being written to, and the parity disk. For these reasons,
the parity disk size must be as large or larger than any of the data disks,
and
given a choice, the parity disk should be the fastest disk in your system."