Many high capacity (>= 64Gb) portable media (microSDXC/SDXC/USB drives) use the exFAT file system these days. A bit of work is required to get things working under Gentoo, hopefully this page will be of some use to others.
This particular file system type is handled under GNU/Linux using the FUSE (Filesystem in Userspace).
You need to enable support for FUSE in your kernel.
File systems -> <*> FUSE (Filesystem in Userspace) support
Save your configuration recompile your kernel and reboot.
You'll need a few filesystem and userspace tools installed too in order to detect, mount and work with FUSE filesystems
emerge -av sys-fs/fuse-exfat sys-fs/udisks
This will also pull in sys-fs/fuse which is a dependency.
Once you've updated your kernel support for FUSE and installed the required tools you're ready to mount the drive. Insert your media into your card-reader (or equivalent) and from a command prompt check what device it is with either dmesg or cat /proc/partitions (it should be the last item listed in the later as you will have only just inserted the drive). You now need to add a line to /etc/fstab to tell your system where to mount the device, I use /mnt/usb2 so have the following in my /etc/fstab
/dev/sdh1 /mnt/usb2 auto noauto,rw,users 0 0
Assuming the device is assigned to /dev/sdh1 you can now issue the following command (hopefully as a user, if not add yourself to the users group in the /etc/group file) to mount the drive…
$ udisksctl mount -b /dev/sdh1 Mounted /dev/sdh1 at /mnt/usb2.
You should now be able to copy files too and from the storage without any permissions problems (something I couldn't work around with a simple /etc/fstab entry).