It is probably already mounted, i guess you are trying to mount partition where your system is installed.
There is a line of information in df -h as follows:
#df -h
Filesystem Size Used Avail Use% Mounted on
tmpfs 7.7G 4.4M 7.7G 1% /
tmpfs 7.7G 0 7.7G 0% /system/bin
/dev/z00IUN4Siy/.magisk/block/system 4.8G 3.1G 1.6G 67% /system/bin/adbd
tmpfs 7.7G 7.4M 7.7G 1% /system/bin/magisk
tmpfs 7.7G 0 7.7G 0% /system/xbin
tmpfs 7.7G 58M 7.7G 1% /gearlock
tmpfs 7.7G 332K 7.7G 1% /dev
/dev/z00IUN4Siy/.magisk/block/sda1 499M 30M 469M 7% /dev/z00IUN4Siy/.magisk/mirror/sda1
/dev/z00IUN4Siy/.magisk/block/sda2 100G 38G 62G 39% /dev/z00IUN4Siy/.magisk/mirror/sda2
/dev/z00IUN4Siy/.magisk/block/sda3 92G 7.9G 84G 9% /dev/z00IUN4Siy/.magisk/mirror/sda3
/dev/z00IUN4Siy/.magisk/block/sda4 30G 11G 19G 37% /dev/z00IUN4Siy/.magisk/mirror/sda4
/dev/z00IUN4Siy/.magisk/block/sdb1 499M 37M 462M 8% /dev/z00IUN4Siy/.magisk/mirror/sdb1
/dev/z00IUN4Siy/.magisk/block/sdb2 40G 15G 25G 37% /dev/z00IUN4Siy/.magisk/mirror/sdb2
/dev/z00IUN4Siy/.magisk/block/sdc1 499M 5.2M 494M 2% /dev/z00IUN4Siy/.magisk/mirror/sdc1
/dev/z00IUN4Siy/.magisk/block/sdc2 41G 10G 31G 25% /dev/z00IUN4Siy/.magisk/mirror/sdc2
/dev/z00IUN4Siy/.magisk/block/sdc4 140G 105G 35G 76% /dev/z00IUN4Siy/.magisk/mirror/sdc4
/dev/z00IUN4Siy/.magisk/block/sdc5 120G 86G 34G 72% /dev/z00IUN4Siy/.magisk/mirror/sdc5
/dev/z00IUN4Siy/.magisk/block/sdc6 120G 107G 13G 90% /dev/z00IUN4Siy/.magisk/mirror/sdc6
/dev/z00IUN4Siy/.magisk/block/sdc7 35G 88M 35G 1% /dev/z00IUN4Siy/.magisk/mirror/sdc7
tmpfs 7.7G 0 7.7G 0% /mnt
/data/media 40G 15G 25G 37% /mnt/runtime/default/emulated
none 7.7G 4.0K 7.7G 1% /cache
sdb2 The partition is android x86 9.0 r2 partition
sda and sdc The partition is a WINDOWS NTFS partition
#mount.ntfs /dev/block/sdc5 /mnt/sdc5
ntfs-3g-mount:mount failed : Device or resource busy
Before installing magisk, i can mount the hard disk with gearlock, and the mount.ntfs command also works!