You'll need more than a ROM to emulate a SCSI harddisk on openMSX 
For that openMSX needs to emulate the SCSI controller on the interface, etc...
IDE is completely emulated and should work out of the box, once you have put the IDE ROM in the share/extensions/ide/roms directory. Start with openmsx -ext ide and proceed like you should when you first boot your MSX with a blank harddisk connected to your new Sunrise IDE interface 
SCSI is not emulated, indeed.
you need a tool called idefdisk.com to create the partitions.
First step is to create (auto partition creation does the trick) them, then initialise them, and you need to write the partition table to disk
After that is done you can restart the system, and copy msxdos2 on the A-drive (and anything else you want to fill your diskspace with 
)
How often do I have to say it:
DON'T USE IDEFDISK.COM!!! USE FDISK310.COM INSTEAD!!!
So, that's clear I suppose... If you'd like to use Okei's FAT16, try the following:
- Autogenerate a partition table
- Write it to HDD ([W] to write, [Y] to confirm)
- Go back to the commandline ([Q])
Type
FDISK310 /FAT16
. This will launch FDISK310 with FAT16 support.
- Then say 'display partition table' ([D])
- Kill all partitions
- Make a small first partition (4 ~ 8 MB or so)
- Mark it with 'B', the boot flag
- Then create as much any-sized partitions as you like, but it's perhaps most useful if you don't make them too big (2GB is the maximum, but I advise you never to choose them bigger than 256MB).
- Put the following line in your AUTOEXEC.BAT file (after you've copied MSXDOS2.SYS and COMMAND2.COM to the 4~8 MB-sized A:\ partition):
FAT009.COM
.
You can utterly replace the '009' in 'FAT009.COM' for the version number you use. I advise you to use the latest (IIRC 0.09) FAT16 driver.
If you'd like to get correct free sizes after a DIR /W command, use the PATCHCOM.COM, created by Manuel Pazos (Guillian on MRC ).
Did you ever bother to read the manual?
openMSX has experimental support for files on the PC's harddisk, but not more than 720KB in one directory and no subdir support. Just use a directory (path) as a disk image and it should work (sort of).