USB host mode not working on Samsung Galaxy S using CyanogenMod
The Galaxy S Plus (i9001) needs an external power source to have a working USB OTG. See this link on XDA for more information (it's for the i9000, but does also apply).

According to GSMArena's spec (USB: Yes, microUSB v2.0) the i9001 firmware doesn't originally support USB-OTG (compare the S3's spec, but 3rd party firmware added this later on. It does however not function without external 5V power.