Adding new disk to LINEAR RAID

Hi there,
About a year ago I've installed OMV to a small PC. For the boot disk I used a CF card connected to SATA via CF to SATA adapter.
Then I plugged two 1TB HDDs and made a Linear RAID using them. Now I ran out of capacity so I bought one more 1TB HDD, plugged it to SATA port on my MoBo, How do I add it to my existing RAID?

When I try to add that new HDD to RAID I get the following:

Select devices to be added to the RAID device

Failed
to execute command 'export LANG=C; mdadm --manage /dev/md127 --add
/dev/sdb 2>&1': mdadm: add new device failed for /dev/sdb as 2:
Invalid argument

Error #4000:
exception 'OMVException' with message 'Failed to execute command 'export LANG=C; mdadm --manage /dev/md127 --add /dev/sdb 2>&1': mdadm: add new device failed for /dev/sdb as 2: Invalid argument' in /usr/share/openmediavault/engined/rpc/raidmgmt.inc:396
Stack trace:
#0 [internal function]: OMVRpcServiceRaidMgmt->add(Array, Array)
#1 /usr/share/php/openmediavault/rpcservice.inc(125): call_user_func_array(Array, Array)
#2 /usr/share/php/openmediavault/rpc.inc(79): OMVRpcServiceAbstract->callMethod('add', Array, Array)
#3 /usr/sbin/omv-engined(500): OMVRpc::exec('RaidMgmt', 'add', Array, Array, 1)
#4 {main}

Zitat von ryecoaaron

Dangerous idea. If one drive fails, you lose all data. I would look at using aufs or mhddfs using the unionfilesystems plugin.

What done is done, now I just need to add more capacity
There's no something important on that RAID, just some music and movies. So if even I lose all of them, I only will need to download them again (but it'll take some time)

Zitat von ryecoaaron

I don't think you can grow linear raid.

Why so?