Had a problem on OS X Yosemite with CH340G chip. Installed the official driver first - http://www.wch.cn/downloads.php?name=pro&proid=178. This didn't work, so I tried to manually load the installed kext and noticed this error in the console:
20/10/2014 00:43:28.492 com.apple.kextd[19]: ERROR: invalid signature for wch.usb.usb, will not load
After a big of googling found this article - http://www.cindori.org/enabling-trim-on-os-x-yosemite/
Just to summarise. On Yosemite:
- install the driver
- Run this:
sudo nvram boot-args="kext-dev-mode=1"
- Restart