As andre says that's pretty ambitious.
1.
I would dispense with the idea of using a USB drive in your "RAID" it would slow your whole setup down and be kind of dangerous from a data standpoint. the two drives you manage to put into your box as a slave and master drive will participate in a RAID ok.
regardless of how you do this you'd be creating a pseudo RAID with
LVM
http://www.tldp.org/HOWTO/LVM-HOWTO/recipethreescsi.htmlor a software RAID using
RAIDTOOLS or
MDADMhttp://www.tldp.org/HOWTO/Software-RAID-HOWTO-5.htmlI'd ask on a debian forum like Andre suggests, as I've never done this before, one Kuro user seems to have done it on the Kuro Forum (I think you'd need an external enclosure and power supply though)
http://www.kurobox.com/forums/viewtopic.php?t=659&highlight=dual+drives BTW 2-
750 GB Seagate PATA Drives would end up giving you a 1.5TB package if you so desired.
2.
Careful with trying to add devices to a software RAID, stick with the two internal drives. and just mount your other drive somewhere inside a directory that belongs to the RAIDED drive. you can use [i]df[/i]
to monitor the space on your devices rather easily.
3,4.
There are quite a few ways to encrypt data onto a drive, if you encrypt the whole block device. like with
Loopaes or
Truecrypt you can't add space to it. If you individually encrypt each file and store it on the block device (like with
ENCFS) then you can make the device as big as you want so long as you manage to get the files back onto the resultant block device
http://www.linux.com/article.pl?sid=06/03/13/1656228http://en.wikipedia.org/wiki/Disk_encryption_software5. Careful with this idea, you loose the key you loose your data, gone kaput, non-recoverable period. (With
ENCFS since I am familiar with this you could come up with a script to look for your USB drive, copy the key file over to the directory where your encrypted data lies.... THEN you can mount the encrypted device. When you leave and unmount the encrypted device you could just delete the key, or to be real anal use a tool like
wipe or bcwipe to do a multipass wipe of the keyfile you copied over.
If you can't come up with this script on your own then you may be better off not trying until you gain a little bit more comfort and experience with writing some shell scripts, and with linux, you can always make this your "last" project in the quest to do this.
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My advice to you would be to stick with RAIDing your two "internal" drives together first with
RAIDTOOLS or MDADM (I think that's how the terastation does it anyways). Once you do that you can use a myriad of options to place an encrypted volume there personally I think the
ENCFS and
Truecrypt options are easiest
A terastation user has already used Truecrypt
http://www.terastation.org/wiki/Encryption%2C_NTFS_Support%2C_and_Windows_Share_ManagementAnd of course I use
ENCFShttp://buffalo.nas-central.org/index.php?title=Encrypted_Filespace_with_EncFS-KP
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... and everything should be encrypted; i read already about encfs, but before i start i have some questions.
sorry no spreken 
