Hi ,
What if External HardDrive could accomadate Led screen to display available space in the drive with a button press ? it would simplify process .The Led display supported by a battery chargable when Drive is in use should not require any additional power .
If you want to check the available space on the drive just press a small button and there u go it shows , regardless of weather its a pendrive or external harddisk the external case should also be put to some use .
Hope this suggetion finds the right person .If the drives(Pendrive/Ext-HardDrives) are already in market please ignore the post :)
Forget just the Available Space info , hard drive are big enough than Ipod screen which lets users even glance through content Overview but that would require battery and hardware used for Ipods .But it would again need password or finger print scanner like laptops have for ensuring only owner may check drive content .