Only showing 1 drive in dashboard?

Probably a very basic question…
I have a 4TB EX2 (2x2TB RED drives). On the dashboard it only shows 420Gb freespace when I know that there is another 2TB free (only backed up my PC which has 2TB drive). Both disks appear in Disk Status - and both are healthy. So Disk 2 doesn’t appear as free space on the main dashboard page. Why?

Is your EX2 in RAID1 mode?

If so then the data is duplicated (mirrored) across both drives, and all you will see is one representative drive (as the content of both will be identical).

And everything is duplicated/written twice, your 4TB capacity will appear as only 2TB (or indeed slightly less than that due to data block sizes). so your 1.55GB or so from your drive is written to each of the 2TB drives in the EX2, hence you only see a remainder of 420GB.

Thanks for the reply - that makes sense.
How do I check whether it is RAID1? and if I want to have 4TB available, do I need to change this to something else - RAID 2 ? I’m unfamiliar with how this all works, as you can tell. I already have another backup running to a WD MyBookLive, so would like to free-up some space on the EX2 to use as a media store too.

There are normally 4 modes available:

RAID0 (Striping) - data is written striped across both drives, with some on one drive and some on the other. Designed for increased performance (speed), but no data redundancy. Capacity will appear as a single 4TB drive.
RAID1 (Mirroring) - data is written in duplicate to both drives at once. No speed increase, but secures against single drive failure. Capacity will appear as a single 2TB drive.
Spanning - Combines the capacities of both drives, to appear as a single larger (4TB) drive. Data is written to one drive and then the other. If one drive fails then data across both can become corrupted, but may be recoverable. No performance improvement or data back-up functionality is provided by this mode.
JBOD (Just a Bunch of Drives) - drives operate independently, and will appear as two separate 2TB drives. No performance improvement or data back-up functionality is provided by this mode.

You can switch between the different modes via the dashboard, but note if you do so it involves formatting the drives, which will erase all existing data on them both.

Thank you for the concise explanation - I’ll have a go at switching to RAID0 sometime soon.