ncp-diag: Falsely reporting data in SD = yes, if root and data directories are BTRFS

My Nextcloud is installed on a small x86-server with Debian using the curl-version of NCP. The root directory / is installed on a BTRFS partition.

I have used nc-datadir to move my data to a plugged in USB drive. Nevertheless, the system info in the NCP Panel, and, of course, ncp-diag still reports that my data are in the SD! Accordingly, system info complains that I should use nc-datadir to move your files to your plugged in USB drive.

Result from ncp-diag:

>>> You should use nc-datadir to move your files to your plugged in USB drive <<<

NextCloudPi version|v1.48.2
OS|Debian GNU/Linux 11. 5.10.0-16-amd64 (x86_64)
automount|yes
USB devices|sda sdb
>>> datadir|/media/USB*** <<<
>>> data in SD|yes <<<
data filesystem|btrfs
data disk usage|213G/932G
rootfs usage|19G/25G
swapfile|/dev/mmcblk1p3
dbdir|/media/USB***
Nextcloud check|ok
Nextcloud version|23.0.7.2
HTTPD service|up
PHP service|up
MariaDB service|up
Redis service|up
HPB service|up
Postfix service|up
internet check|ok
port check 80|open
port check 443|open
IP|192.168.***
gateway|192.168.***
interface|enp1s0
certificates|***
NAT loopback|no
uptime|1:11

I took a look at ncp-diag. In line 33 the location of the data directory is tested:
echo "data in SD|$( [[ $( stat -fc%d / ) == $( stat -fc%d "$DATADIR" ) ]] && echo yes || echo no )"

If I understand it correctly, the stat command is used to compare the file system at the root directory / against the $DATADIR directory, where the Nextcloud data are stored. It compares the number of free inodes in each filesystem: If the numbers are identical data in SD = yes. Otherwise the result is no.

In my case, both directories, the root directory and the $DATADIR directory, reside on a BTRFS partition. BTRFS does not use inodes for its meta data. Accordingly, it will always report 0 as the number of free inodes!

Wouldn't it be better to test directly, if both directories are on the same device? This could be done via findmnt:

echo "data in SD|$( [[ $( findmnt -n -o SOURCE --target / ) == $( findmnt -n -o SOURCE --target "$DATADIR" ) ]] && echo yes || echo no )"

findmnt is part of the package util-linux, which, according to my knowledge, is automatically installed in Debian as well as in Raspbian/Raspberry PI OS.

Start at line 33 of ncp-diag, where stat compares the root and data-directory filesystems and incorrectly reports data in SD for BTRFS. Review the proposed findmnt-based comparison, then verify that the diagnostic reports no when the directories are on different devices and yes when they share one.