If you are going to use jumpers, both drives have to be jumpered
If the ST360020A will be the Slave, remove all jumpers. (You already
know this, since you mentioned it above.) Also, be sure it is connected
to the Slave connector on the IDE cable. Speaking of the IDE cable, I
would use an 80-conductor cable rather than the older 40-conductor
type, although that should not be a total barrier to operation.
Regarding the Master, then, be sure it is jumpered in a way that allows
the Slave to be present, and be sure the Master is connected to the
proper connector on the IDE cable. No hints on proper jumpering
because you did not provide the model number.
Before trying to boot with the new configuration, and especially since
there is apparently a problem, be sure to enter the BIOS configuration
screen and make sure both drives are properly recognized, (make,
model, size, and master/slave status), before proceeding.
If all of that is good and the system will not boot, provide details on
exactly what happens. Boot progress, error messages, etc. There is a
slim possibility that the system's boot files are on the drive that
was removed, but you can see if that is the case by looking at Disk
Management. Not sure if you have already checked that.