Complete newbie - installing old HDD

soltarian wrote:
Also, I want to use the 2nd disk purely to augment the capacity of my readynas duo; not for back-up using RAID.

As you guessed, you will need to factory default your unit and select RAID 0 as the RAID-level: http://home.bott.ca/webserver/?p=425

soltarian wrote:
Now I know I probably need to format it but I don't know how (and in which format)... If I do a factory reset of my ReadyNas Duo, will I lose the data that I have on my 2TB disk in slot1?

Yes, you will lose your data so you'll need to back it up to external USB. Depending on how you're using your NAS (backup only, primary storage, etc.) you may want to consider getting an external USB for regular backups of the NAS, as a single drive failure could result in data loss on both drives: http://www.readynas.com/?p=3153

soltarian wrote:
... I want to add another disk (1TB) in slot2; this disk was used as an external disk on a macbook and already has some files on it. When I put it in slot2, it recognizes it but I can't use the drive.

When re-configuring your NAS, you will need to install the smaller capacity drive in slot 1. Bear in mind that any data on the drive will be nuked, as the NAS will re-partition and format it for use with the Duo. Make sure that the disk is listed on the HCL: http://www.readynas.com/?page_id=82

You may also want to test it using vendor tools: http://home.bott.ca/webserver/?p=388

-Dave