Windows storage spaces

Hi

If I created Windows storage spaces with optimization between two usb hard drives, can I use single one of them to copy some files to it or I must always have both hard drives connected at the same time ?

Both at the same time or there is no 'pool' of devices. The Storage Space info is written to all the disks in the array regardless of the setup, so even an array of two disks with one as parity needs data written to both at the same time.