I'm not entirely sure about BR discs if they can be backed up yet? I hadn't any problems copying my divx file over. It plays just fine

1 file, no, the PS3 only supports Fat32, so if the file is > 4GB, you won't be even copying it to the Fat32 formatted Memory card.  Most 720p movies are at least 4.3GB, so NOPE, can't use NTFS.  This is COVERED by the way, SEARCH...

Now, you specifically said play a BR-D movie from the flash drive, so how are you copying the PROTECTED movie to the flash drive?  IT has been done, you need the right software to rip the disc, which is going to be HUGE, to the HDD first. You will end up paying MORE for a 16GB memory card that it's worth.   If you want full 1080p, 8+GB file size for a MKV as well, that's converted...

Using a MEDIA SERVER you can get the 1 file to the HDD, from the XMB with the copy option, then sit and wait.  Once it is on the PS3 HDD, you can't copy it FROM there to anywhere else, only play and delete.