[NTLUG:Discuss] OT: ATA Drive Question

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I purchased a used IBM Netvista computer with a 20 gig ATA drive (according
to the part number).
When using ubuntu's disk partitioner or using Partition Magic, I only see 10
gigs.
If this is in fact a 20 gig drive, How do I make the other 10 gigs visible?

Thanks,
Bradley