Hello,
I have read many pages of this forum but did not see something to help me.
If this issue has already been answered, thanks to redirect me with a link 
After playing a while with a linux distro (AmiPUP 1.1.3), I decided to add it to my laptop.
The laptop already runs plop 5.0.15 (MBR) and Win7 (/dev/sda2).
According to GParted, partitions are as follow :
/dev/sda1 : laptop diag partition
/dev/sda2 : ntfs - win7
/dev/sda3 : extended
/dev/sda5 : ntfs - win7 swap
/dev/sda6 : ext2 - AmiPUP
unallocated
/dev/sda7 : ntfs - win7 new laptop drivers
According to the "Puppy Universal Installer", the installation of AmiPUP went successful (/dev/sda6).
According to the "Grub4Dos boot-loader config", the installation of Grub4Dos went successful on the "Partition Boot Sector" (PBS) of /dev/sda6 ("Do not rewrite the existing boot record" box ticked).
After reboot, on plop menu, I have added a new AmiPUP profile and linked the third line on the "linked partition" menu.
So the third line of "HDA" is now : *80H DON'T TOUCH L2.
To my understanding the second extended partition is referred by plop as L2 (which is on Grub: /dev/sda6).
I am expecting the following actions :
Turn on PC -> MBR (plop) -> /dev/sda6 (Grub4Dos) -> AmiPUP
And now, why I wrote all those lines 
When I try to boot AmiPUP, the following message is displayed on the screen :
LOAD PROFILE DATA
MBR: CHECK
MBR: IS VALID
CHECKING BOOTSECTORWARNING:
NO VALID BOOTSECTOR SIGNATURE
CONTINUE BOOT? [Y/N]By pressing "Y", screen gets black and cursor blinks on the top left corner of the screen.
and nothing...
I have tried with L1 (NTDRL missing error message), L3 and L4 (same NO VALID BOOTSECTOR message), but nothing helped.
Does anybody have an idea on how to get Grub4Dos loaded ?
Did I miss something ?
Thanks in advance.