Using 2 existing HDD (3TB)

Not at all. What you cannot do is take an existing volume and make a "larger" one from it. If you are creating a Mirror volume, you can create a second mirror volume with the new drives.

If you want a single larger volume, like a RIAD 4/5/10, then you need to back up and create the larger volume.

Not at all. What you cannot do is take an existing volume and make a "larger" one from it. If you are creating a Mirror volume, you can create a second mirror volume with the new drives.

If you want a single larger volume, like a RIAD 4/5/10, then you need to back up and create the larger volume.

Noted. Thank you for your advice.

Let say I have 4 x 3TB = 12TB. Using SoftRAID 5 I get only 9TB. Not sure if I can create 2 volumes: 4.5 TB for daily use and data, the other 4.5 TB to use for backup or time machine.

Is this possible? Please advise, thank you.

You can create as as many volumes as you like, just specify the size when you create a volume.

Hope you also have a second backup strategy, backing up to the same disks is not really backup. ;-)

You can create as as many volumes as you like, just specify the size when you create a volume.

Hope you also have a second backup strategy, backing up to the same disks is not really backup. ;-)

Oh that's true. I am using OWC thunderbay 4. How about using 3 x 3TB for SoftRAID and get 1 x 4TB for Time Machine to back up data under SoftRAID?

Hope this methods can work perfectly!

As long as Time Machine volume has the space, as your 6TB volume is bigger than the Time Machine volume.