High hard drive activity on lock screen

[Protec]

I noticed that when my computer is locked the hard drive/s are at 100% utilisation and the hard drive activity LED is on all the time. I would expect this if the drive was the C drive but this is a hard drive with just documents / media on. Anyone know what Windows is doing while on the lock screen to explain 100% utilisation.

[ZeroHour]

Search indexing because the PC is idle?

[FN-GM]

Anti Virus scan?

[Oaktech]

Or updates?