diff --git a/scripts/backup b/scripts/backup
index 76d788a..6530d47 100755
--- a/scripts/backup
+++ b/scripts/backup
@@ -63,9 +63,10 @@ for (glob("/backup/*")) {
 }
 
 
-# find the backup dir with the most free space available.
-# other strategies are possible, like round robin, or daily, weekly,
-# etc. backups.
+# Choose a random backup directory.
+# The directories are weighted by free space (e.g, if we have two
+# directories with 3 TB and 2 TB free space, then their chances of being
+# chosen are 60% and 40%).
 
 my @backup_dirs = map {
 			$_ = $1 if m{(.*)/.*}; # basedir and detaint
@@ -82,7 +83,10 @@ my @backup_dirs = map {
 my $sum_free = 0;
 $sum_free += $_->[1] for (@backup_dirs);
 $ca->log(3, "total free est. $sum_free bytes");
-my $rnd = rand() * $sum_free;
+my $rnd = rand();
+$ca->log(3, "random (raw) = $rnd");
+$rnd *= $sum_free;
+$ca->log(3, "random (scaled) = $rnd");
 my $count_free = 0;
 my $backup_dir;
 for(@backup_dirs) {