`fopen` cannot create a binary file on NAS

I've been using fopen on a NAS regularly without a problem, but somehow I encounter a problem today, and fopen keeps failing to create a binary file.

fid = fopen(outfilename,'w');

fid =

-1

K>> outfilename

outfilename =

'\\nasdrive\.....\yyyyyy\xxxxxx.bin'

folder = fileparts(outfilename);
isfolder(folder)

ans =

logical

1

 fileattrib(folder)

Name: '\\nasdrive\.....\yyyyyy'

archive: 0

system: 0

hidden: 0

directory: 1

UserRead: 1

UserWrite: 1

UserExecute: 1

GroupRead: NaN

GroupWrite: NaN

GroupExecute: NaN

OtherRead: NaN

OtherWrite: NaN

OtherExecute: NaN

Apparently, the folder exists and I have write access to the folder (UserWrite = 1), but I cannot create a file (fid = -1). The file name (2020-11-09_12-09-41_snr1a_01_iron.bin) doesn't contain illegal letters.

For comparision, I tried save and it doesn't work either.

 save(fullfile('\\nasdrive\.....\yyyyyy','temp.mat'),'args')

Error using save

Unable to write file \\nasdrive\.....\yyyyyy\temp.mat: permission denied.

I'm using MATLAB R2020b and Windows 10.

Does anyone has a clue of how to fix this issue??? Why I cannot write a file even though UserWrite = 1? I have another folder on the same NAS, which has the same output for fileattrib, and fopen works in that folder.

[Kouichi C. Nakamura]

I wondered if the file path was too long. But it seems unlikely. The file path was 138 letters, much shorter than Windows 10's limit of 260.

https://uk.mathworks.com/matlabcentral/answers/224771-why-matlab-won-t-support-file-name-length-more-than-63-char

> No, Matlab supports longer file names. Some commands are limited by the file system, e.g. deleteinmg files to the recycle bin under Windows suffers from the horrible limit of 260 characters for the path name. But for a simple file copy etc. wide names are accepted, which allows 32767 characters.

[Image Analyst]

Looks like a permission issue rather than a MATLAB issue. Can you use File Explorer and right click in that folder and say "New => text document"? Does that work?

[Kouichi C. Nakamura]

Uh, thanks! Now I figured out where I made a mistake! I've confused the two drives one of which is read-only and the another with write access. It was a simple human error. Not a MATLAB issue after all.