Hello Tapeworm,
following your tip, I have installed a micro SDXC 64 GB class 10 Sandisk card (FAT32 formated) in my Sansa Clip Zip and everything seems to work fine, both in Rockbox and original firmware.
This is just what i wanted!!!
Thank you!!
PD: if I ever get my hands in a 128 GB SDXC, i will report the results.
