I can switch it to Mass Storage USB. But all that shows up is a "Emulated CD" (like you get on some USB Sticks) and it has HTC Sync Install Kit.
You can't delete it or anything.
How can I access my SD Card Via USB Mass Storage?
Testing with a 32GB microSD card I have seemed to work. It initially came up in "HTC Sync Manager" (MTP) mode, but tapping the "USB File sharing" notification and selecting "Disk Drive" caused 2 USB Storage devices to appear, one CD-ROM containing the HTC Sync Manager software, the second storage device was the microSD card (which was automatically unmounted on the phone).