O.T: Hard disk size not recognised. I would appreciate your collective wit and wisdom to help me

Question

I would appreciate your collective wit and wisdom to help me solve this one: I have a failing Maxtor One Touch HD in a box I carry with me on my travels. Thinking I'd be really clever I bought a Seagate Barracuda 7200.10 750 GB ATA100 disk to replace it. That way I can carry all my junk with me. No matter what I try the disk only reports as 137 GB. I can format and partition, but only 137 GB. I have 500 GB (SATA) disks in my PC that are recognized OK. I've downloaded the SeaTools util from the Seagate web site, but that does nothing useful. (I did not know that Seagate and Maxtor are one and the same. Does that mean the same quality of HD's?) Any useful suggestions will be gratefully received. Thanks.