RAID 1 and a weird external HDD problem

hiya

having some strange HDD issues with my macbook pro,
I have a 400gb external which has been working fine for about 2 years, but now it takes a while to mount- about 5 minutes, but weirdly it goes dead slow when it's first mounted (system profiler confirms it's mounted as usb 1.0 11mb/s) but after 20 minutes it clicks into usb 2 and runs at the full 480mb/s
the drive works at full speed when taken out the enclosure, and if the caddy is plugged into a powered usb hub it works right away (at full speed), so my only guess is the power supply?

Raid 1- had a hard drive failure the other week (did manage to save all the data thanks to data rescue 3) and it got me thinking about raid- I know everyone hates on raid 1 because it "won't keep your data safe in case of a power spike, or if a giant lizard comes down from space and smashes your drives into dust"- but it's better than nothing, and nothing is exactly what I have right now
so
i'm thinking 2tb lacie (because it looks pretty...), but rather than set it up as a 2x1tb raid 1 setup, to set it up so each drive is independent, where each drive has a 750gb and a 250gb partition, then make nightly ghosts of the 750gb partition onto the other drive, and use the 250 gb partitions as photoshop/final cut scratch disks and time machine backup (for my 160gb internal drive) respectively- is this retarded, or a good use of space

p.s. for a photoshop scratch disk, would I see better performance off a firewire 800 drive set up like I plan to, or to upgrade my internal drive to a 7200rpm?

also, is there any plugin/hack which allows folder merging- it's the one thing I miss from windows, my music collection got screwed because of the lack of merge! had to rerip 30gb of CD's

thanks!