|When they're connected to the same data cable, either both drives need to be jumpered Cable Select, or one drive needs to be jumpered Master, the other Slave.|
The jumper settings are shown on the label on the top of the drive.
Some IDE hard drives, e.g. many Western Digital models, have two ways the drive can be designated as (jumpered as) Master - e.g. Master, single, or similar, for when the drive is by itself on an IDE data cable, and Master, with Slave, or similar, for when the drive is on a data cable with another IDE hard or optical (CD or DVD) drive.
That setting must be correct for the situation.
OR - you can set the single drive on a data cable to CS - Cable Select - and have it on the end connector of a 3 connector data cable, or set both drives on the same data cable to Cable Select.
Dell™ Dimension™ 4550 Series