BusyBox's tar --exclude and -X option do not appear to be working (version 1.26.x)

[gestos]

Hello to all,

for my first post on this list, an issue I am experiencing with BusyBox 
1.26.x version of tar (mipsel architecture). I want to dump a system 
image from a combo USB3.0 / WiFi / Ethernet hard drive. The OS seems to 
reside on a small flash chip, while the storage drive is internally 
connected by USB and mounted at /data. Since the current system seems to 
be using a customised version of BusyBox 1.12 where options have been 
removed, I simply downloaded a more recent version in a user-accessible 
folder and called the newer binary instead.

command is exactly:
./busybox-mipsel tar -czf 
/data/UsbDisk1/Volume1/Share/Direct/Images/System.tar.gz / -X /data

But I also tried, without success:
./busybox-mipsel tar -czf 
/data/UsbDisk1/Volume1/Share/Direct/Images/System.tar.gz / -X=/data
./busybox-mipsel tar -czf 
/data/UsbDisk1/Volume1/Share/Direct/Images/System.tar.gz / --exclude=/data
./busybox-mipsel tar -czf 
/data/UsbDisk1/Volume1/Share/Direct/Images/System.tar.gz / --exclude=/data/*

However, for some reason this command doesn't exclude the /data 
sub-tree, and instead starts on generating an enormous System.tar.gz 
file, which is obviously not the correct one.

What is the exact syntax for excluding a given sub-tree?