reactos-bootcd-0.4.12-dev-835-gc1fbc2d-x86-gcc-lin-dbg is the guilty rev.
My virtual disk is about 1GB more or less.
Partition 0 uses 256 mb and has an ext2 fs (active)
Partition 1 uses the remain space (around 768 mb)
I made a linux installation on Partition 0, Linux distro is MicroCore 4.4.7
Partition 1 was previously formatted as FAT32 with mkdosfs from linux.
When installing the guilty rev, Usetup performs a check to the partition 0 (active) to see if is possible to install the bootlader.
At this point Usetup fails to detect properly the partition 0 filesystem, instead of returning EXT2 it returns RAW
Then I choose the partition 1 to install ReactOS, but then Usetup requests a format for the partition 0, and gives no other choice than formatting the partition and thus breaking the entire existing linux installation.
This is bad as ReactOS currently cannot be installed as alternate OS with dual boot because otherwise it will break others OSes in the same disk.
Looking deeper into code and in the logs, I found up the clue.
In my case Usetup is hitting this condition:
If anyone wish to test the same disk I've used, here is uploaded.