I'm trying to move some mp3 files from Drive C to Drive D, which is where
I keep all my music files.
These files are pretty large, 162MB and up.
All was fine, but suddenly now when I try to move them,
this pops up in the system tray:

And the files won't move over to the D Drive.
According to the info in Disk Management, the D Drive capacity is 42.93GB,
and the free space is 23.40GB.
The music folder I'm trying to move them into has 16.5GB of mp3/ogg files in it.
I can move small files over, like jpg, png, native psd, even small mp3's.
Just not these larger files.
And there's no problem moving them from D to C.
Any ideas on what's keeping me from moving these files over,
and what I can do to correct this problem?