Critical Failure - Volume 1 degraded

Sorry but newbie here.
I have a 6 TB RAID configured device about a year old.
All has been set up ok but recently I had “Low Temperature " warning every few seconds (it lied, everything was warm!)
Rebooted and one RAID volume rebuilt - albeit slowly!
Today I have the same Low Temperature” Notification again and also:
" Volume Degraded
Volume 1 degraded. Please check status of drive by running Disk Test in Settings >> Utilities.
Thursday, 2018 November 22, 8:28:46 "

So, what now please

Thanks Bob

Oh yes, just forgot.
Ran the system test quick version and after about two hours, it hadn’t finished. Was I premature in cancelling it?

What specific My Cloud device do you have? This subforum generally discusses the single bay/single drive My Cloud units which look like this:

These single drive/single bay My Cloud devices do not have multiple internal hard drives configured for RAID.

If you have a multi bay My cloud unit see the relevant subforum for your model where users more familiar with that device may be able to assist or where there may be past discussion (found using the forum search feature, magnifying glass icon upper right) similar to your issue.

Thank you, doI repost over there or?