disk space 100%

Hello,
I have the omv 2 installed on an HP thinclient with a flash memory of only 2GB. This thinclient does not support a hard drive bigger than 16 GB on the SATA interface, and even if I put a HD in this interface, it does not boot through it, only via flash card.
In summary, I would like to run OMV on this 2GB flash card, but it constantly reaches 100% usage and I can not find which folder or file is occupying unnecessary space.
What is the minimum disk space required?