I have swapped the original 5Gb drive in my portable drive with a 20Gb. This worked however for some reason I can only adress 10Gb in 2gb partitions (due to the limitation of FAT). Not sure why I can only address 10Gb but I have not looked very deep for that. The 5Gb went bust so I had to replace it.
I assume the larger portable drives use FAT32 so you might be limited to only 2 partitions of 30Gb.
So, yes it can be done but I cannot tell you exactly what possible configurations you will end up with.