i would go with the 2nd, since you could have it outside your case and connect your power and Sata cable out the back, so you can remove it quickly, if you want a permanent one got for the first if you have space in your PC
I didn't say earlier.. but I already have a removable SATA drive in a docking bay which is basically a mirror*
As well as the SATA docking bay, the PC also has a front SATA connector port.
However with the "loss" of 2 machines in the same week, it means that I have a few spare IDE drives.. I'd like to use the 250GB one and install it permanently inside the new machine... but everything on the mobo is either SATA or eSATA.
*Bit long winded to describe the whole backup stuff. I was at one point thinking RAID but after much thought I have another process which involves a combination of Ghost, Robocopy and Replicator and a networked LinkStation.. together with the removable drive means that I have something more suitable to my requirements and hopefully more robust that what RAID alone could do.