Thanks for the feedback.

Here's a better patch.

I've left out the usage of err for now, because it's not consistently
used in that script.

Index: sysupgrade.8
===================================================================
RCS file: /build/data/openbsd/cvs/src/usr.sbin/sysupgrade/sysupgrade.8,v
diff -u -p -r1.13 sysupgrade.8
--- sysupgrade.8	8 Jun 2022 09:03:11 -0000	1.13
+++ sysupgrade.8	14 Jun 2025 10:23:50 -0000
@@ -99,3 +99,6 @@ Directory the upgrade is downloaded to.
 .Nm
 first appeared in
 .Ox 6.6 .
+.Sh BUGS AND LIMITATIONS
+.Nm
+currently does not support downloading the sets to a NFS or MFS partition.
Index: sysupgrade.sh
===================================================================
RCS file: /build/data/openbsd/cvs/src/usr.sbin/sysupgrade/sysupgrade.sh,v
diff -u -p -r1.52 sysupgrade.sh
--- sysupgrade.sh	19 Jun 2024 05:22:33 -0000	1.52
+++ sysupgrade.sh	14 Jun 2025 10:21:06 -0000
@@ -132,6 +132,11 @@ fi
 
 install -d -o 0 -g 0 -m 0755 ${SETSDIR}
 cd ${SETSDIR}
+if df -t nfs,mfs ${SETSDIR} >/dev/null 2>/dev/null
+then
+	echo "Error: ${SETSDIR} on NFS or MFS is currently not supported"
+	exit 1
+fi
 
 echo "Fetching from ${URL}"
 unpriv -f SHA256.sig ftp -N sysupgrade -Vmo SHA256.sig ${URL}SHA256.sig