I need to change Hdd Scsi capacity

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Hi to all,

I have more old Hdd IBM DGHS COMP IEC-950.
This hdd are 9Gb capacity.
I need to change its capacity from 9Gb to 4Gb or 2Gb.
I need the SCSI adapter see this Hdd not for 9Gb but for 4Gb or 2Gb.
I suppose i need to Low Level Format, use some Scsi command, or modify Hdd geometry, but i do not know how.
Someone can tell me how i can do that ?

Thank you to all