Allows again to use a single data disk with single parity.

raid.h

@@ -59,6 +59,7 @@
 /**
  * Minimum number of data disks.
  * Some optimizations require at least two disks.
+ * It's anyway allowed to have a single data disk with a sigle parity.
  */
 #define RAID_DATA_MIN 2
 

state.c

@@ -895,7 +895,7 @@ static void state_map(struct snapraid_state* state)
 		exit(EXIT_FAILURE);
 	}
 	/* some raid functions assume at least two data disk */
-	if (diskcount < RAID_DATA_MIN) {
+	if (state->level > 1 && diskcount < RAID_DATA_MIN) {
 		fprintf(stderr, "Too few data disks. You must define at least %u.\n", RAID_DATA_MIN);
 		exit(EXIT_FAILURE);
 	}