I'm not sure Picasa would find photos on the hard drive if they have been deleted either manually or by means of moving them to a drive which hasn't the capacity to hold them.

If Picasa isn't successful, try using >>> Restoration (click here) <<<

Once installed, double click on the Restoration icon then select the Local Disk "C" in the Drives menu, although this should be selected by default.

In 'All or part of the file' put whatever format the photos were recorded in such as 'JPEG' or 'JPG' (try both) then press the Search button.

Should it find them, highlight each one, then press 'Restore by copying'. This is preferably done to another drive (it will warn you of this) so you don't over write any of the remaining items your trying to recover.
Your not limited to a HDD. You can use removable drive such as the one you were trying to move then originally from as another drive.
However, don't use that 'stick' as we now know it's not big enough. You can also run Restoration again with your 'Stick' inserted and select that in the drives menu, then use the same method as above.

May or may not help. Worth a try.

You can also try doing a 'Search' via Start > Search > Pictures, Music or Video > Tick Pictures and Photos > In all or part of the file name, either input the file name, or part of it if you renamed them, or as above 'JPEG or JPG.
You'll be surprised how many it finds. View in Thumbnail mode to see the items it finds. Be aware that the Thumbnail mode really slows down the process. Be patient.

Good look.