Mac automatically goes to sleep (despite sleep being set to never) when trying to upload data to internet or external hard drives.

So I have a problem....

We use a computer as a server at work. It has 400GB of data on it, and as it is an old machine, I want to back it up.

I have tried 2 different external hard drives – both of which make the computer go to sleep after 15 mins, despite this being set to Never is preferences.

This isn't limited to hard drives, as I have tried transferring the data online – same issue.

It's worth noting that when the computer is in normal use, it never goes to sleep, just in the instances described above.

I have also tried using Caffeine, which is supposed to prevent computer from sleeping, to no avail. I have also deleted the file

/Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist

All software is up to date.

Any advice?