Click Type then select Partitions, although this greys the other options out, so not sure if I needed to do this.
With Option "Use existing AROS partitions (on any drive)", you saying to the installer: "Look for the correct partition by your own". So you do not need to say, where it's located - so the other options become greyed out.
The Device field is still blank, so I typed this in, again I tried with Unit of 0 and 1:
Switching between Unit "0" and "1" has effected nothing, because you saying with "1", that GRUB has to be installed on another drive.
Just short: Drives in AROS are listed as numbers ("0", "1", "2", "3" and so on) , like in Windows letters ("A", "B", "C", "D" and so on). If you saying your first drive, you would say in Windows "C", in AROS "0". If you saying your second drive, you would say in Windows "D", in AROS "1". If you saying your USB-Stick as mass storage, you say in Windows "E", in AROS "2".
Back to point:
Could you check, which partition type your drive have? AROS/Icaros cannot use "GUID Partition Table" (GPT), so it muss run on "Master Boot Record" (MBR). If you have GPT on your drive, you will have to wipe full disk (and delete CloudReady). Opening HDToolBox again and double click the white field. When double clicked, you should see your drive. Select your drive to see on the left side your partition table.