Because AntiVirus is probably scanning the files
And Windows is using lots of memory
And the USB HardDrive may be fragmented or even in need of a check Disk
It's hard to tell.
You could try in Safe Mode (F8 Repeatively during system startup)
Then go to My Computer and try again
If it fails in Safe Mode, it's probably:
Faulty Ram: run
Memtest for 7 Passes
Faulty HardDrive run CheckDisk on the drive (right click-Properties-Tools)
Or it can be Pagefile set too low (Right click My Computer-Properties-Advanced...)
I'd also recommend
CCleaner (in Normal mode again) and Defrag both drives