About switching to 3.55 CFW...

Hello Everyone.
I have a Slim 120gb PS3 running a groopic v4 hermes on 3.41 firmware.
I've been tempted about this all new 3.55 CFWs arpund, specially since online playing seems to be possible.

I wonder if there's any chance to make something like this.
Keeping my PS3 original hdd with 3.41 (as it is right now).
Buying an external 1TB HDD with esata. Format it as internal, load 3.55 CFW.
And switch firmwares just switching the main HDD.

I apologize if it sounds weird, trying to do my best english here.
I also searched the forum for similar threads and I haven't found any.
Thanks for reading.

Doesn't work that way. Once you upgrade, it re-keys your HDD with a new encryption code. So, you won't be able to just upgrade/downgrade with HDDs.

Well, that was fast! Thanks