Error 87 = The parameter is incorrect.
This can relate to logical locations rather than physical location, may be related to windows shortcut names
As Pondus said, is this an external drive.
As Error 21 = The device is not ready. This would be unusual if it weren't an external drive location.
When you say your not sure if this is external or not, I would suggest that it isn't as having a second hard drive, external to the main system by a USB connection (etc.) would be obvious.
In your AvastUI > Scan > select the scan you did - scroll down to - Scan Settings > Sensitivity > Links.
Do you have the 'Follow links during scan' option selection ?