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.