scsi to usb adapter for external hard drive

You have a lot of challenges...
- it is unclear whether on-board SCSI controller can talk to large capacity SCSI drives. The link you provided lists both SE and DE drives.
- it is unclear whether that ancient HP UX can talk to large drives.
- you will be hardly find large-capacity SCSI drive these days.

Your system' performance is probably on par with modern-day tablet or RaspberryPI. The project you are using this on is on the verge to colllaps any single day - hard disks do fail, computers do fail, and it is not question of whether, but when.