It should be noted that the current version of the
PDC does not support having a BDC in the domain. Because of this,
if the PDC fails, there is no way for Windows Client users of the
domain to be authenticated. And, if a disk fails on the PDC, there
is no backup on the domain with the critical credential data. This
means that it is very important to make backups of users credential
files. It also means that there is no system that can be easily
promoted to a PDC to take the place of the existing PDC