Have two partitions in RAID1

RAID can be used to mirror whole disks or just partitions, though support varies depending on which RAID method you use.

Hardware RAID cards tend to mirror only whole disks. After creating a mirror of a whole disk you can create multiple partitions on that new virtual disk.

Windows software RAID seem to mirror partitions. This does not prevent you from creating two mirrors, one sided 100GB and one sized 1.8TB. Just realise that this might not automatically clone the boot sector or non-obvious partitions, resulting in problems [such as this one].

Linux software RAID (MDx) can do both volumes and whole disks (The result of handling both whole disks and partitions as files). You can even use md to create a new RAID array without loosing old data if you do it like this.

For more detailed information we would need to know which OS you are using and/or which RAID method. Software RAID via the OS, fake RAID (Intel), HW RAID (and which model).

Could you add this information to your post? And comment on this? (The latter so I get a ping and I can update this 'comment' posted as an answer (due to lack of proper markup and length restrictions in comments). Alternatively drop in on our generic chat room. It is not the place for live support but if I am active in it I can answer in a lot more detail.