as it's already been mentioned, except my advice is clear some space on the main or a secondary internal fixed device and don't use backup, external or NAS drives for anything other than data, games aren't classed as data either
whiles is possible and easy to do, eg moving a data folder like mydocs to anywhere, M/S doesn't recommend moving it to anywhere other than a fixed device, eg a permanent drive, a NAS or backup drive isn't permanent if it can be unplugged or powered down differently than the main system drive,...
Most consumer NAS drives have very slow CPUs cycles and lack dedicated RAM, which also won't do you any favors long term regarding access to games access, plus as you have already found out, unless you map the drive to permanently use the same letter and make sure no other device which randomly plugs in can use it once the NAS is offline or disconnected, any data or game installed on the NAS can or could have an issue if needed to be upgraded or patched etc.
so unless you have money to burn and have a top end NAS, just buy a bigger main drive, which will work,.
< Message edited by zakblood -- 9/21/2018 4:18:18 PM >