Thank you for your advices.
I followed them and got help from Rod Smith (creater of gdisk).
The problem came from the fact that Easeus (the partition tool I used) was not aware of "hybrid MBR/GPT" systems. It did a mess thus.
Here were the steps to be taken :
1) using gdisk I repaired the GPT :
- added the missing Mac partition (command 'n' in gdisk stating the type AF and the begin and end position)
- sorted the partitions for it to become the second one as expected (command 's' in gdisk)
2) I have recreated the hybrid MBR based on these data ('x' command and then 'h' command in gdisk).
Of course, Gdisk allows also to create a backup of both partitions tables. That is a great tool and Rod's documentation on his website rodsbooks.com is great too (with examples and so on).
If you have a dual boot Mac with Windows that is great to correct your messed up partitions.
Turns out this problem is caused by trying to install Windows on a GPT disk with USB install media formatted with MBR.
Thanks to a suggestion by @magicandre1981, I created my USB install media with rufus, taking care to select the "GPT partition scheme for UEFI devices" option, booted into the USB drive in UEFI mode, and was able to install Windows properly.
Best Answer
GPT and MBR are disk partitioning formats. They allow a single physical disk to be split into smaller partitions (volumes)
Basic disk is a plain simple GPT or MBR disk. Each partition on a basic disk spans a linear continuous region on the disk. Dynamic disk OTOH is an abstraction layer above that so it can be constructed on either GPT or MBR formats. It allows the creation of logical partitions that can be easily extended (expand in size), spanned multiple empty regions on a single or multiple physical disks or a software RAID volume for speed or data integrity...