Well, I've tried the following file DS3612xs_3202-Repack.vdi, I replaced it by removing synoboot.vdi and when I boot I get the following message:
GRUB Loading stage1.5.
GRUB loading, please wait...
Error 22
FYI: I created DS3612xs_3202-Repack.vdi with VirtualBox command line from DS3612xs_3202-Repack.img
VboxManage convertfromraw DS3612xs_3202-Repack.img DS3612xs_3202-Repack.vdi