SD card readers don't work via USB OTG but USB thumb drives do, any ideas? I have a Huawei Ascend G6 LTE (G6-L22) and bought a little USB OTG cable so I could read full sized SD cards while out and about.

Only, it doesn't work, on either of the SD card readers I've tried - both of them small USB2 based readers.

When I plug in USB thumb drives to the adapter, they work fine and I can browse files and photos.  But no go on the card readers.

What will happen is the phone either doesn't see the drive at all, or it sees it momentarily then it disappears with a "your SD card was disconnected" notification.  The LED on the card reader lights up when you first plug it in, but then goes into a fast blinking pattern which seems to indicate a problem.

Any ideas?  I wouldn't have thought an SD card reader would exceed the normal power supplied via USB OTG ports, and its LED lights up.  Could the phone just not have a driver that supports card readers?

Edit: the filesystem on the SD card was FAT32, as it was a 16GB card I use in my DSLR.  I use the stock ROM but am rooted.
I believe that the problem is lack of power. Your phone doesn't deliver enough power to the card reader. I would suggest using the same kind of cable you used to need when connecting an external hard disk using USB. It has one contact for the hard disk (card reader in your case), one for the phone and one for extra power (use a separate USB charger/adapter).  That should solve the power problem.

I don't know where one can find this kind of cable in your part of the world. They should be fairly easy to find though.

If this doesn't work I would use the Googles ADB developer tool to check the logs on the phone to see if there are any error messages that could help.

Good luck!