device in raidz2 array only partially recognised by the OS

(posted to both -current and -fs because am unsure which is more relevant)

On a machine running GENERIC-NODEBUG amd64 1500026 1500026 FreeBSD 15.0-CURRENT #0 main-
n273486-88dd0550920c, a disk in the raidz2 array dropped out and is no longer recognised by
the OS in terms of zfs but *is* recognised by smartctl. I've tried onlining the device but
it says it can't find /dev/da7. The array is connected in HBA mode to a 
HP Smart Array P440 Controller. The disk dropped out of the array after 
an extended power outage.

zpool status:

   NAME                    STATE     READ WRITE CKSUM
   data                    DEGRADED     0     0     0
     raidz2-0              DEGRADED     0     0     0
       da1                 ONLINE       0     0     0
       da2                 ONLINE       0     0     0
       da3                 ONLINE       0     0     0
       da4                 ONLINE       0     0     0
       da5                 ONLINE       0     0     0
       da6                 ONLINE       0     0     0
       114031416858974629  UNAVAIL      0     0     0  was /dev/da7

da0 is the boot disk and is not part of the array. It's not a root-on-zfs.

smartctl:

doas smartctl -x -d cciss,7 /dev/ciss0

smartctl 7.4 2023-08-01 r5530 [FreeBSD 15.0-CURRENT amd64] (local build)
Copyright (C) 2002-23, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Vendor:               SEAGATE
Product:              ST4000NM0023
Revision:             A001
Compliance:           SPC-4
User Capacity:        4,000,787,030,016 bytes [4.00 TB]
Logical block size:   512 bytes
LU is fully provisioned
Rotation Rate:        7200 rpm
Form Factor:          3.5 inches

(etc etc)

There are no relevant errors in the iLo. How can I make the zpool see the disk again?
--