diff --git a/README.md b/README.md
index 136be2d..c5d820a 100644
--- a/README.md
+++ b/README.md
@@ -131,9 +131,20 @@ between keepalives, lower it.
 - Only one process may talk to the serial port. The script takes an exclusive `flock` on
   `/run/das_fanctl.lock` and exits rather than interleaving garbage with another writer.
   Detach `attachDAS.sh`'s `screen` session before starting the service.
-- The disk glob is `/dev/disk/by-path/*-sas-exp*-lun-0` — anything behind a SAS expander,
-  i.e. the shelf. The host's own drives hang off `pci-…-scsi-*` and are not matched. A
-  second enclosure would be lumped in with the first.
+- **`DISK_GLOB` must name the shelf's HBA on any host with more than one.** The default
+  `*-sas-exp*-lun-0` means "behind a SAS expander", which is not the same as "in the
+  shelf": a server's own backplane is frequently behind an expander too. On `iz-pve0` the
+  internal drives sit on `pci-0000:02:00.0` and the shelf on `pci-0000:04:00.0`, so the
+  default glob matched all 17 drives and would have set the shelf's fans from the
+  server's internal drive temperatures. Set
+  `Environment=DISK_GLOB=/dev/disk/by-path/*04:00.0*-lun-0` in the unit.
+- **The server's own fan controller needs the mirror-image setting.** Whichever host the
+  shelf is plugged into must set `DISK_EXCLUDE_PATTERN` to that same HBA in
+  `fan_speed.service`, or the shelf's drives drive the *server* fans as well. Moving the
+  shelf between hosts means moving both settings.
+- `PORT` is per host too. Check which UART actually answers rather than assuming:
+  `grep -v uart:unknown /proc/tty/driver/serial` lists the real ones, then send
+  `_temp_rd` to each and watch for the `BlueDress` prompt.
 
 ## Files
 
diff --git a/das_fanctl.sh b/das_fanctl.sh
index 5443502..aa28e75 100755
--- a/das_fanctl.sh
+++ b/das_fanctl.sh
@@ -7,7 +7,7 @@
 
 source /root/fan_speed/functions.sh
 
-PORT=/dev/ttyS1
+PORT=${PORT:-/dev/ttyS1}
 LOG=/root/fan_speed/log/das_fan.log
 CHECK_INTERVAL=30              # seconds between temperature readings
 KEEPALIVE_INTERVAL=5           # re-send _shutup this often, the DAS forgets it otherwise
@@ -16,7 +16,11 @@ HIGH_FAN_SPEED=60
 
 # Disks in the enclosure - anything behind the DAS SAS expander. The server's own
 # drives hang off pci-...-scsi-* instead, so this glob does not pick them up.
-DISK_GLOB='/dev/disk/by-path/*-sas-exp*-lun-0'
+# Drives in THIS shelf. "-sas-exp" alone is not enough to identify an enclosure:
+# a server's own backplane is often behind an expander too, and on iz-pve0 the
+# internal drives sit on pci-0000:02:00.0 while the shelf is on 04:00.0. Name the
+# shelf's HBA explicitly on any host with more than one.
+DISK_GLOB=${DISK_GLOB:-/dev/disk/by-path/*-sas-exp*-lun-0}
 DISK_LOW_TEMPERATURE_THRESHOLD=40   # at/below this -> LOW_FAN_SPEED  (idle spread is 32-39c)
 DISK_HIGH_TEMPERATURE_THRESHOLD=50  # at/above this -> HIGH_FAN_SPEED