If it were possible to use the same USB stick with another operating system, for example to store files to use on Windows, a virus in the other operating system could corrupt your Tails.
To store files in your TailsUSB stick, use the [[Persistent Storage|doc/persistent_storage]]. To exchange files between Tails and another operating system, use a separate USB stick.
If you worry that your Tails might be corrupted, do a [[manual upgrade|upgrade]] from a trusted operating system to upgrade it to a trusted version of Tails.