diff --git a/README.md b/README.md
index f8796bb..997c552 100644
--- a/README.md
+++ b/README.md
@@ -4,37 +4,54 @@ This is currently in prototype status.
 
 ## Requirements
 
-apt-get install sg3-utils
+Requires Python and the `sg_ses` utility, provided by [the `sg3_utils` package](http://sg.danny.cz/sg/sg3_utils.html).
+
+Debian-based systems might use:
+
+    # apt-get install sg3-utils
+
+On RHEL/CentOS systems:
+
+    # yum install sg3_utils
+
+FreeNAS 9.10 includes `sg_ses` as part of the standard image.
 
 ## Usage
 
-find your scsi controller (one of the /dev/sgX devices, perhaps using lsscsi)
+Find the SCSI Enclosure Services device corresponding to your SA120. On Linux, this is likely one of the `/dev/sg*` devices. (Try `lsscsi`?) On FreeBSD, this is likely `/dev/ses*` instead.
 
-change /dev/sgX in fanspeed.py if needed.
+You can use `sg_ses` to identify SES devices, e.g.:
 
-````
-# python fan.py 2
-Fan 0 speed: 0
-Fan 1 speed: 947
-Fan 2 speed: 932
-Fan 3 speed: 812
-Fan 4 speed: 932
-Fan 5 speed: 947
-Reading current configuration...
-Setting fan 0 to 2
-Setting fan 1 to 2
-Setting fan 2 to 2
-Setting fan 3 to 2
-Setting fan 4 to 2
-Setting fan 5 to 2
-  LENOVO    ThinkServerSA120  1007
-Sending Enclosure Control [0x2] page, with page length=296 bytes
+    # sg_ses /dev/ses0
+      LSI CORP  SAS2X28           0717
+    …
+    # sg_ses /dev/ses1
+      LENOVO    ThinkServerSA120  1007
+    …
 
-Fan 0 speed: 0
-Fan 1 speed: 945
-Fan 2 speed: 932
-Fan 3 speed: 812
-Fan 4 speed: 926
-Fan 5 speed: 945
+Once you've found your SA120, use `fancontrol.py` to set the fan speed:
+
+    # python fancontrol.py /dev/sg0 2
+    Fan 0 speed: 0
+    Fan 1 speed: 947
+    Fan 2 speed: 932
+    Fan 3 speed: 812
+    Fan 4 speed: 932
+    Fan 5 speed: 947
+    Reading current configuration...
+    Setting fan 0 to 2
+    Setting fan 1 to 2
+    Setting fan 2 to 2
+    Setting fan 3 to 2
+    Setting fan 4 to 2
+    Setting fan 5 to 2
+      LENOVO    ThinkServerSA120  1007
+    Sending Enclosure Control [0x2] page, with page length=296 bytes
+
+    Fan 0 speed: 0
+    Fan 1 speed: 945
+    Fan 2 speed: 932
+    Fan 3 speed: 812
+    Fan 4 speed: 926
+    Fan 5 speed: 945
 
-````