After partitioning the disk with an external tool, I tried to let the reactos installer format the disk. This caused an error in
The following parameters were used:
Resulting in BootSector.FATSectors32=10231, which results in the (fatal) error "Filesystem has 1309567 clusters but only space for 1309566 FAT entries" later during installation.
The current calculation is would be perfect if all entries of a fat table are usable. But the first two entries are not, see (1).
Attached a patch which simply verifies the calculation, and add a sector to the fat table if required. The attached logfile and spreadsheet are just for reference.