download does not resume if drive letter is changed

I started to download a file to a UFD at drive F then I paused the download and tried to resume later in a different computer where the UFD was assigned to drive G

The download failed with the following message:

F:\Documents\Videos\sample.f4v could not be saved, because an unknown error occurred.
Try saving to a different location.

Is it a bug or download portability is not supported?

Both computers are Windows 7 Ultimate, FF 3.6.10

Thanks