after upgrade to Yosemite my ExpressCard reader did no longer work with my Lexar CF Reader

Me too. I've been waiting for some kind of update from Lexar but so far, nothing. I called them and the girl on the phone was not very helpful at all. I depend on this stuff for my living. I guess I'm going back to USB 2.0 on my MBP 17" 2010.

yep. I had to do a painfully slow usb 2.0 transfer of a 64gb card direct from my camera.

after that I went directly to eBay and purchased a firewire 800 lexar reader. PLEASE FIX THIS

This is absolutely ridiculous. Same thing here. Crappy companies treat their customers like crap. Was to be expected from a company that still hasn't upgraded their one and only "professional" monitor for 3+ years...

Vive l' iphone!

I Found that I could download in smaller batches (About 270 images from a Canon 5Dsr) serious pain in the proverbial and hope they fix it soon.

The problem is the kext is unsigned, and is being blocked by OS X.

If you want to enable it, you can run the following in terminal to allow unsigned kexts, however be aware of the risk it creates.

sudo nvram boot-args="kext-dev-mode=1"

After a reboot, your CF card reader will work.