Before installing openmediavault make sure your hardware is supported.
CPU: Any x86-64 or ARM compatible processor
RAM: 1 GiB capacity
System Drive: min. 4 GiB capacity (but more than the capacity of the RAM)
Data Drive: capacity according to your needs
The whole disc will be occupied by the system and swap space , so size doesn’t matter so much. Data storage on the system disc is not supported.
If you use a Flash Drive, select one with static wear leveling , without this the drive will have a very short lifetime. It is also recommended to install and activate the Flash Memory plugin. The entire disk is used as system disk and can not be used to store user data.