There's two problems I have with this solution, one is that by doing that, I'd be pissing away the 4G of internal memory I paid for when I purchased my 930, and the other being: would I get a performance hit by moving the OS onto an external SD card?
A much nicer solution is for everything to be pooled and seen as one memory cache. Get your thumb out, TomTom.
You will not encounter a performance hit by moving everything to the SD card as long as you use a decent speed SD card. I use the Sandisk Extreme II 4GB SDHC card and everything runs beautifully! Here is the link to the card I bought. I actually bought mine at walmart for $15
http://www.newegg.com/Product/Product.aspx?Item=N82E16820171347
I didn't realize you had a 930 and was thinking you only has 2 GB of internal memory. In your case you could possible by an 8GB but that kinda would defeat the purpose of you having any internal memory. And I also forget that you have 2 maps.
On my device, since I only have 2 GB internal memory, I moved everything from the internal to the 4GB SD card. This also benefits me because when Home goes to update anything, it won't ever have any space limitations, and if I install a new map, I won't have voice recognition disabled, which Home normally does because 2GB isn't enough for everything.
On the 2GB internal memory, I store my MP3's. I also store many MP3's on the remaining space on my SDHC card..
The choice is really up to you. I would think only the EU map would be on the SD card and everything else resides on the internal memory, therefore you shouldn't have any problems when it comes to using Home to update your device. You should only have to selece "internal memory" if you aren't actively using the EU map.
As far as pooling the 2 memories together, PC's can't even really do that. Between 2 thumbdrives or hard drive and thumbdrive, they will always appear as separate devices. I'm surprised that the TomTom does as good a job as it does when it comes to combining the MP3 folders on both my SD card and internal memory. I see what you mean though, that when you open Home, you shouldn't have to select between the 2.