Porteus

I have no problem with grub legacy.

With an external porteus system on usb, why don't you reformat the deficient partition with gparted and then install grub legacy to the mbr of this second disk of yours, see first post of thread on grubconfig:
http://forum.porteus.org/viewtopic.php?f=62&t=946

phhpro wrote:Another painless option is grml-recueboot to boot straight off the ISO.

I support the booting ISO concept. Grub4DOS and Easy2Boot will boot Porteus no problem.

Please stay on topic. The OP wants an answer to his problem and not the preferences of others.

My BIOS has no problems with drives above 137Gb

Can you boot a different OS from this same position on the hard drive to confirm? Shouldn't take long to test.

ng system is placed above 137Gb? I was able to boot to the operating system that covered almost the whole 320Gb drive.

brokenman, do you mean if the operating system is placed above 137Gb? I was able to boot to the operating system that covered almost the whole 320Gb drive.

I mean the start of the partition. From your fdisk output it appears that the partition starts at a point above this limit.

I've only got two partitions on the second SATA drive: a small swap partition at the start and a 300Gb partition that contains Mandriva which I can boot to.

Are you positive about Porteus supporting to live somewhere close to, or beyond the edge of physical hardware boundaries?

Perhaps you misunderstood. Around 7 years ago many BIOSs were struggling with the ATA interface limit which sits around 137GB. Since I do not know the age of the hardware of the OP it was an option to look at. It has nothing to do with Porteus specifically but rather a limitation at a bios/hardware level. When I saw sdb5 I made an assumption. Carry on, it seems this is not the issue.

I would advise installing as phppro says. Simply. Copy porteus files to your preferred partition. Add Porteus to whatever existing boot manager you have. Nothing more needs to be done.