Format incompatibility. CM 10.1 does not support the exFAT or NTFS filesystems required to make effective use of SDCards >32GB.
Let me explain:
SDXC MicrosSD cards now come in capacities over 64GB and, as shipped, phones like the Samsung Galaxy S4 supports them in hardware and software.
Capacities of over 32GB are very trouble-prone when using the mroe common FAT32 file system found on lower capacity SDcards. Worse, Windows absolutely refuses to format a +64GB SDCard to FAT32. Samsung (and I am sure other vendors that support 64GB SDCards) have licensed Microsoft's exFAT filesystem to legally provide support for exFAT. However CyanogenMod does not have this licensing in place and Google does nto provide blanket licensing for it.
This is a royal pain to those of us that have a 64GB SDCard in our Sammy GS4s, but want to move to SM10.1 or related rom. This leaves either reformatting your +64GB SDCard as FAT32 using a third party utility OR downsizing to a 32GB SDCard if you want to run anything except the bloated, OEM-branded, as-shipped ROM.