I want to connect the second VDI (3,78GiO). How I do that ?
If I release the current VDI and then select back gentoo2.vdi a new differential VDI is created. This is why we can see 3 lines at the same level on the picture.
OK, I have found. I have to attach the differential .vdi to the HD controller instead of the original .vdi. The differential .vdi was in Snapshots directory.