Bios flashing : copying exe to bootable external hd (caot create regular file, read only filesystem)

I want to flash my bios so I can reset it with HP super+b boot bios reset. I downloaded the .exe flash. I created a Windows PE iso and bued it on a exteal HD that I will boot on, using the 'dd' command. I now want to copy the .exe on the exteal hard drive.

cp sp64081.exe /run/media/nano/Microsoft Windows PE (X86)/

cp: caot create regular file '/run/media/nano/Microsoft Windows PE (X86)/sp64081.exe': Read-only file system

With -f, same output. I'm root. I first installed freeDOS on the exteal HD, and was able to copy the file and boot, but the .exe bios flash file caot be executed in DOS mod T.T (that's why I'm using windows PE).

The HD is NTFS. I use arch (manjaro JWM up-to-date). When I use GUI filebrowser to change 'write permissions' from read-only to write allowed :

error: read-only filesystem

When I mount /dev/sdb (= exteal hard drive) :

mount: /dev/sdb is write-protected, mounting read-only

When I

chmod 777 /run/media/nano/windowspe/

Output is

chmod: changing permissions of '/run/media/nano/windowspe/': Read-only file system

Must have done a stupid thing somehow. Thanks in advance