[910H] TitaniumBackup Can't write to SDCARD after upgrading to 5.1.1

Normally this would be fixed by editing platform.xml in /etc/permissions, but if I try to edit that file I get an error that says the entire filesystem is read-only. The app has already been granted root access, so I'm not sure if this is an issue with root, or perhaps there's something else I'm missing.

I followed the instructions in this thread to upgrade to 5.1.1, then I installed WanamLite from this thread.

Any pointers?