Implementing a "sometimes" RAID on a laptop. (eSATA, SSD, RAID 1)

[linux guy]

I experienced a complete SSD failure this week on my laptop.

I've ordered a new Dell XPS 17 laptop which has an eSATA port.

Given the nature of the SSD failure I experienced, from now on I wish
to have my laptop running a RAID1 setup via the eSATA port when its
used on my desk.   However, when its not used on my desk, I wish it to
function normally without the RAID functionality.

Question 1.

How does one set up a "sometimes" RAID ?   Or would something like
rsync be better ?   What happens the first time I plug my laptop into
the eSATA cable after being away from my desk ?  What happens if there
is both new data and an error in existing data ?  How does the RAID
software know the difference ?

Question 2.

Internally, my XPS17 has 2 hard drives.   I will probably use an SSD
for the OS and a 750 GB 7200 RPM conventional drive for data.

How does one configure the single external eSATA RAID drive to back up
(mirror) the data for both internal drives ?

Question 3.

The OS drive will be an SSD which is faster than the eSATA RAID drive,
which will probably be a 7200 RPM 2TB+ conventional drive.  Will this
limit the speed of the SSD to that of the eSATA drive or is buffering
employed to allow one to be faster than the other ?

Question 4.

Can only a portion of the eSATA RAID drive be allocated to the RAID
and the rest left to be mounted by the laptop for general access ?

Thanks !