Be sure your BIOS has been updated to allow large sized drives. Usually this should be not a problem for anything smaller than 137 GB, but on some computers they have other limits built into the BIOS. If your drive is larger than the BIOS allows, you will need to update the BIOS by a download from the computer manufacturer or the motherboard manufacturer.
Are your hard drives similar in type and brand? Some hard drives do not play nice together. If the 40 GB is an ATA-33, or 4200 rpm or other older drive, you may not get detection of a new ATA-100 or ATA-133 7200 rpm... particularly if they are made by different manufacturers.
You can also have a problem if one drive is formatted as Fat32, and the other is NTFS on some models of computers or setups.
You may have a jumper problem, as this is the most common fault.
If using cable select, your main drive must be on the end plug, with the second plug in the secondary drive socket.
But I prefer to use the jumpers as they are described on the back of each drive. Your main or 0 drive would be jumpered as Master. Your secondary or 1 drive would be jumpered as Slave.

Then if your power plugs from the power supply are working correctly everything will show up in the BIOS, and both drives will be detected at boot.
If they are detected at boot, and still not accessible, check your EIDE ribbon cable. They are fragile and are easily damaged from crimping, folding or wire pull-outs from the plug at either end.

And finally, if the Seagate is not properly setup using the Seagate disc or software, the problem you describe can occur. You can download the software, if you bought the hard drive as OEM and it did not arrive with the blue Seagate disc.
Good luck. Let us know how it turns out.