Have no knowledge of Dell.. however,
if you can gain access to the System Bios, you will see that the system defaulted to a hard drive parameter, therefore you must set that drive to CDROM... Some Bios allow AUTO setting..
I would take it that you have installed the Toshiba on Secondary controller..
if not and you only have one Hard drive, then you have a conflict of devices..
Check hard drive for jumper, denoting Master, Slave, or Cable Select (CS).
then you must set the cdrom drive to (CS) and insure it is on the end of cable.
However, I find it best to set the hard drive to Master, and the CDROM to Slave if on the same controller.. and not use the (CS) at all.
Hope this helps