You need to feed that SSF file to the Differntial Correction module to produce a COR file...you should then be able to transfer that COR file to you unit and edit/update the file.
SSF files are data that have been downloaded from the GPS unit tou your PC. they package all of the assorted files that your GPS uses so there are fewer files to lose accidentally- like a ZIP of your GPS data. They are indeed a backup of what was on there, however I am not surprised that it returns as unusable when you transfer it back to the GPS. If the SSF file contains GPS information from a few years ago, as you say, and it has has not been processed to a COR file (this is how to refine the accuracy of multiple GPS observations to actual GPS'd points), you'll be unable to edit it just due to the age. I think that GPS data files cannot contain satellite observation data spanning more than 1 week. So for every week of collection, in my experience, you need to differentially correct the GPS data to a COR file and send that COr file back to the GPS for the next week
I've had trouble with .CAR files myself....not sure what exactly they are or how their absence can mess up data transfer (though I have observed that they can cause an issue). As best I can gather, though...the car files might contain the carrier signal information that would have been packaged into the SSF on transfer. If indeed they do contain carrier signal data, they would need to be regenerated and replaced every time you transfer the data to you GPS as they become obsolete. I bet it will be back there if you go out and collect some points.