You need to write the OMV install image to the USB stick and instruct the PC to boot from USB. To avoid problems you may also want to disconnect all other drives before attempting to install OMV. When OMV is installed you can reconnect the drives and share their content using OMV. All this is the normal procedure when installing OMV. Except that you typically want to wipe the drives you will use as data drives.
Also note that the performance of OMV may be very disappointing if you use a windows native filesystem like NTFS and/or connect data drives via USB.
OK, I can disconnect them before installing but if USB stick (HDD) has already the image to be installed, where it will be installed? On the same USB stick / HDD from where the OMV will be installed?
Regarding the performance, I just want to check if I am able to do some of the settings I want to do in OMV before buying dedicated equipment.
I am now thinking to buy Synology DS218+ vs Odroid which is much cheaper.
BTW, can I install it on a WDMyCloud storage device? this is the current cloud version which is not working properly anymore.
Thanks.