Ferjo was it a second Hard-Drive you added? if so and you added it to the secondary slot (slave) on the Primary IDE cable then you need to make sure that the setting for BOTH hard drives are correct.

IE, the Primary Drive has its jumpers set as MASTER, and the secondary (backup) drive is set as slave.

Alternativley make sure both are set to auto, if these setting are in any way off-set they could cause problems??