I tried this command :
/vmfs/volumes # vmkfstools -P "/vmfs/volumes/DATASTORE"
VMFS-5.54 file system spanning 1 partitions.
File system label (if any): DATASTORE
Mode: public
Capacity 53418655744 (50944 file blocks * 1048576), 52400488448 (49973 blocks) avail
UUID: 51c99974-3e0e9fd0-40e4-90e2ba309b92
Partitions spanned (on "lvm"):
naa.600605b00581fb6018c09f852081693a:3
Is Native Snapshot Capable: NO
Here, the blocks are 1Mo (1048576).
Like you said, it seems that the read-only error occurs because i try to create on a occupied space.
But I dont understand how to seek the new partition in the free space.
Any help is welcomed