Hello hupvik,

When you do a firmware update you would need to call in to Support team and open a ticket.

An engineer will then do a remote session and run health checks on the system to makes sure a system is good for upgrade.

Once they confirm health checks look good an update rule will be created for the system to reach out to our SupportAssist servers to download.

I recommend to call in to support to work with an engineer. If you don't have an active warranty you can ask about a Post Standard Support contract.

There are many factors can cause write cache disabled.  Generally from DELL support team , they are first to ask to upgrade your SCOS. Actually we can did some troubleshoooting to find the write cache disabled root cause. 

SCOS not available in Drivers & Downloads