it is not,
I have tested it, reformatted it, recopied the files.
let me reformulate the issue as clear as possible :
before each attempt, here is the status of the cards
CARD A : Sandisk 128Gb class 10 with 13422 mp3/flac/ including very few m4a/wma/ogg... every tracks tags have been checked and filled up.
card is ExFat, every unwanted filetypes have been removed (*.db, *.mpc, *.flv, *.mp4 etc) and .audio folder has been removed.
.jpg and .png files are present in folders.
card has been scanned in a card reader to avoid errors and safely ejected from computer.
CARD B : Sandisk 128Gb class 10 with 9585 mp3/flac/m4a/wma... every tracks tags have been checked and filled up.
card is ExFat, every unwanted filetypes have been removed (*.db, *.mpc, *.flv, *.mp4 etc) and .audio folder has been removed.
.jpg and .png files are present in folders.
card has been scanned in a card reader to avoid errors and safely ejected from computer.
I used batch tool with firmware 1.3.3 and 1.4.2, in both case, it resulted in a impossible full scan of the second card scanned,
with the difference that with FW 1.3.3 ogg files were properly recognized so I managed to have a first card scanned with no (Null) artist.
right after a fresh reset, I insert card in the stated mentionned above ;
if I scan card A first, in slot 1, leaving slot 2 empty,I manage to have 13422 songs recognized
same if I scan card A in slot 2, leaving slot 1 empty.
same result if I insert card A & B but choose to scan only slot in which is card A.
then whenever I try to scan the slot in which card B is inserted, the scan never finishes and I hear a clicking noise as if the device just turned off, but the screen is actually on and back on home page.
now the same is valid the other way around, means if I scan card B first, the scan finishes properly with 9585 track added, but I'm unable to scan card A afterwise.
the only option left to me is format the cards into FAT32... but I'm tired of copying files and scanning now.