come access this problem too.
attach USB HDD to computer
Open Computer Management (start --> run --> compmgmt.msc)
Under storage select Disk Management
if you see you're drive listed as type 'Basic' and 'File System' (colume header) as NTFS then delete the partition and recreate it but shoose the open to NOT format it.
Format the HDD under WBFS
You can verify it by going back into Computer managment and you will see you're HDD listed as Type = Basic and File system as RAW.
note - this will of couse wipe your HDD and you will need to readd your games again.