Hello!
Instead of duplicating my script, I tried another solution.
I made a big script who get all necessary values and use SendToModule to set each part of my FvwmButton.
The script:
WindowTitle {scriptShowSize.fvwm}
WindowSize 105 12
Font "Shadow=0:xft:M+ 2p:size=8:bold:antialias=True"
Colorset 13
Init
Begin
# Call to the script
Set $cmd = {exec sh $FVWM_USERDIR/seed/scripts/showsize.sh }
# Partitions
Set $sda1 = $cmd {/dev/sda1}
Set $sda2 = $cmd {/dev/sda2}
Set $sda5 = $cmd {/dev/sda5}
Set $sda7 = $cmd {/dev/sda7}
Set $nfsMulti = $cmd {earth:/Qmultimedia}
Set $nfsDown = $cmd {earth:/Qdownload}
# Get value
Set $osda1 = (GetOutput $sda1 1 -1)
Set $osda2 = (GetOutput $sda2 1 -1)
Set $osda5 = (GetOutput $sda5 1 -1)
Set $osda7 = (GetOutput $sda7 1 -1)
Set $onfsMulti = (GetOutput $nfsMulti 1 -1)
Set $onfsDown = (GetOutput $nfsDown 1 -1)
# Set My button title
Do {SendToModule "place" ChangeButton dd1 Title "} $osda2 {"}
Do {SendToModule "place" ChangeButton dd2 Title "} $onfsMulti {"}
Do {SendToModule "place" ChangeButton dd3 Title "} $onfsDown {"}
Do {SendToModule "place" ChangeButton dd4 Title "} $osda5 {"}
Do {SendToModule "place" ChangeButton dd5 Title "} $osda7 {"}
Do {SendToModule "place" ChangeButton dd6 Title "} $osda1 {"}
End
PeriodicTasks
Begin
If (RemainderOfDiv (GetTime) 60)==0 Then
Begin
# Get value
Set $osda1 = (GetOutput $sda1 1 -1)
Set $osda2 = (GetOutput $sda2 1 -1)
Set $osda5 = (GetOutput $sda5 1 -1)
Set $osda7 = (GetOutput $sda7 1 -1)
Set $onfsMulti = (GetOutput $nfsMulti 1 -1)
Set $onfsDown = (GetOutput $nfsDown 1 -1)
# Set My button title
Do {SendToModule "place" ChangeButton dd1 Title "} $osda2 {"}
Do {SendToModule "place" ChangeButton dd2 Title "} $onfsMulti {"}
Do {SendToModule "place" ChangeButton dd3 Title "} $onfsDown {"}
Do {SendToModule "place" ChangeButton dd4 Title "} $osda5 {"}
Do {SendToModule "place" ChangeButton dd5 Title "} $osda7 {"}
Do {SendToModule "place" ChangeButton dd6 Title "} $osda1 {"}
End
End
Widget 1
Property
Position 0 0
Size 105 24
Type ItemDraw
Flags NoReliefString Left
Main
Case message of
SingleClic :
Begin
End
End
And my button look like this:
#------------------------------------------------------------------------
# Place
#------------------------------------------------------------------------
Style "place" !Title, !Handles, Sticky, CirculateSkip, StaysOnBottom, FixedPosition, !Iconifiable, WindowListSkip
DestroyModuleConfig place: *
*place: Geometry 240x425+0+220
*place: Padding 0 0
*place: Colorset 13
*place: Rows 31
*place: Columns 8
*place: BoxSize fixed
*place: Frame 0
#*place: Left
*place: Font "Shadow=0:xft:M+ 2c:size=9:bold:antialias=True"
*place: (2x4, Icon place/stock_folder.png)
*place: (4x2, Title " /root")
*place: (2x4, Icon place/go.png, Action(Mouse 1) Exec exec $[filemanager] /)
*place: (4x2, Id dd6, Title " ")
*place: (8x1, Title " ")
*place: (2x4, Icon place/home.png)
*place: (4x2, Title " /home")
*place: (2x4, Icon place/go.png, Action(Mouse 1) Exec exec $[filemanager] /home/shaika-dzari)
*place: (4x2, Id dd1, Title " ")
*place: (8x1, Title " ")
*place: (2x4, Icon place/network_local.png)
*place: (4x2, Title " nfs://Download")
*place: (2x4, Icon place/go.png, Action(Mouse 1) Exec exec $[filemanager] /mnt/earth-download/)
*place: (4x2, Id dd2, Title " ")
*place: (8x1, Title " ")
*place: (2x4, Icon place/network_local.png)
*place: (4x2, Title " nfs://Multimedia")
*place: (2x4, Icon place/go.png, Action(Mouse 1) Exec exec $[filemanager] /mnt/earth-multimedia/)
*place: (4x2, Id dd3, Title " ")
*place: (8x1, Title " ")
*place: (2x4, Icon place/folder-video.png)
*place: (4x2, Title " /Multi")
*place: (2x4, Icon place/go.png, Action(Mouse 1) Exec exec $[filemanager] /mnt/multi/)
*place: (4x2, Id dd4, Title " ")
*place: (8x1, Title " ")
*place: (2x4, Icon place/folder-downloads.png)
*place: (4x2, Title " /Torrent")
*place: (2x4, Icon place/go.png, Action(Mouse 1) Exec exec $[filemanager] /mnt/torrents/)
*place: (4x2, Id dd5, Title " ")
*place: (8x2, Swallow scriptShowSize.fvwm `FvwmScript $[FVWM_USERDIR]/seed/scripts/scriptShowSize.fvwm`)
Work great! 
The only thing I was unable to do is to align the text inside my button to left.
If I use *place: Left, my FvwmButton crash. No big deal.
Thanks again for your help Thomas 
Regards.