These questions are related to AD and FSMO roles:
1- If I need to reboot a DC do I need to transfer the roles to another DC or the transfer is automatic when the DC is no longer online? would this have any impact ?
2-Can someone explain to me clearly why Infrastructure role should not be in the same DC that is a Global Catalog? Is this not recommended only when you have more than one domain or even within one domain?
3- How do you seize a role from a DC that has crashed?
4-if the PDC emulator has a wrong time, would other computer be able to logon to the domain?
However, if you're formatting and wiping a DC, you MUST transfer the FSMO roles off of it and demote it gracefully using dcpromo prior to formatting.
Placing the Infrastructure Master role on a Global Catalog is not supported, since a GC holds a partial copy of every object in the forest. That server will therefore not act correctly as an Infrastructure Master since no records will be updated by the server. This does not apply in either a single-domain forest, or in an environment where ALL DCs are also Global Catalog servers.
Seizing FSMO roles: http://www.petri.co.il/seizing_fsmo_roles.htm. Also, remember to do a metadata cleanup of a failed DC.
By default, no machine will authenticate with a DC if the time is out by more than 5 - 15 minutes. It is the time difference between the machine being used at login and the authenticating DC which is used to determine whether there is a change.