Just wanted to mention that I am experiencing the exact same thing as the original poster,
The disk drive holding the local cache is filled up despite using a fixed cache size.
I am running StableBit CloudDrive 22.214.171.1247 BETA on Win7 x64 Professional.
The same happens on all 3 drives (all Amazon Cloud Drives ).
When detaching the drives, the exact amount of data in total written to the drive in question is then "released" on the cache drive. I just detached one of the drives releasing 519 GB of data (the fixed cache size for the drive is 10 GB ).