Cleaning up Samsung S2 memory When I use the application manager, I see that 1.8GB are occupied out of 2GB available. This of course causes issues when installing apps and brings up a warning notifications that some system services may malfunction. However, if I sum the memory taken by my apps, I come up at 1GB, and many of these apps have been moved to the SD card.

I've tried to use Clean Master to free space, but without much success.

So I want to cleanup files manually. What partitions on the phone's system are the ones in the 2GB space? Is it the root partition?
/storage/sdcard/android is probably where all the data is.  You can manually delete files out of there, but I would recommend using the application SD Card Maid.  I say that just because it has a lot of functions to clean up data from apps that are no longer installed.