Is it safe to use windows on a usb to hide the fact that my computer is running qubes os? This config also allow me to use windows for some tasks. The usb disk isn’t a windows installer, it boots into windows directly, will this comprise the qubes safety?
Short answer: Yes.
If entire qubes installation encrypted, in such way you may compromise /boot sector of qubes that can’t be encrypted.
Windows also may clone encrypted copy of your Qubes disk installation and allow attacker to unencrypt it for future access to your Vault for example.
If entire installation not encrypted at all or attacker manage to clone it, look at:
focus on: Emergency backup recovery (v4) | Qubes OS
While for unencrypted case, attacker may replace parts of Qubes installation, for example leave in your system infected copy of any template or infect dom0.