Permissions to the DNS-321 or from the DNS-321 should match the drive. Normally this means using the user 'EVERYONE'. At least for backing up to the DNS-321 from VISTA I have the user name set to 'EVERYONE' with random password. This works fine for my weekly backup.
I also backup using Robocopy and for what you want to do it should work fine as seen from the test run below.
Backups from the DNS-321 NAS to an external hard drive using Robocopy
C:\utils\nas>REM Copy from DLINK NAS to external hard drive 'L:'
C:\utils\nas>robocopy "\\dlink-59c1b6\Volume_1\Media\Informational" L:\FOOBAR /MIR /FFT /Z /LOG:test.log /TEE
Log File : C:\utils\nas\test.log
-------------------------------------------------------------------------------
ROBOCOPY :: Robust File Copy for Windows
-------------------------------------------------------------------------------
Started : Tue Feb 02 02:12:03 2010
Source : \\dlink-59c1b6\Volume_1\Media\Informational\
Dest : L:\FOOBAR\
Files : *.*
Options : *.* /FFT /TEE /S /E /COPY:DAT /PURGE /MIR /Z /R:1000000 /W:30
------------------------------------------------------------------------------
New Dir 1 \\dlink-59c1b6\Volume_1\Media\Informational\
100% New File 18.5 m Spending The Last Of Your FSA.mp4
New Dir 7 \\dlink-59c1b6\Volume_1\Media\Informational\Garmin Nuvi 265WT GPS\
100% New File 31.3 m Garmin nuvi 265WT - ComputerTV Review.mp4
100% New File 129.2 m Garmin Nuvi 265WT - during flight.mp4
100% New File 8.4 m Garmin nuvi 265WT - Features Tutorial.mp4
100% New File 37.3 m Garmin nuvi 265WT - HSN - 10-25-2009.mp4
100% New File 7.9 m Garmin Nuvi 265WT - Review.mp4
100% New File 74.2 m Garmin Nuvi 265WT - Traffic Reporting Part 1 .mp4
100% New File 21.8 m Garmin Nuvi 265WT - Traffic Reporting Part 2.mp4
New Dir 2 \\dlink-59c1b6\Volume_1\Media\Informational\Microware\
100% New File 1.5 m Mark Hawkins at PennFest 2000.mp4
100% New File 9.1 m Tandy Color Computer Easter Egg.mp4
------------------------------------------------------------------------------
Total Copied Skipped Mismatch FAILED Extras
Dirs : 3 3 0 0 0 0
Files : 10 10 0 0 0 0
Bytes : 339.57 m 339.57 m 0 0 0 0
Times : 0:01:04 0:01:04 0:00:00 0:00:00
Speed : 5563088 Bytes/sec.
Speed : 318.322 MegaBytes/min.
Ended : Tue Feb 02 02:13:09 2010
Checking the external drive
C:\utils\nas>dir L:\FOOBAR /S
Volume in drive L is ExtMedia
Volume Serial Number is 3844-63FB
Directory of L:\FOOBAR
02/02/2010 02:13 AM <DIR> .
02/02/2010 02:13 AM <DIR> ..
02/02/2010 02:13 AM <DIR> Garmin Nuvi 265WT GPS
02/02/2010 02:13 AM <DIR> Microware
12/23/2009 01:28 AM 19,450,648 Spending The Last Of Your FSA.mp4
1 File(s) 19,450,648 bytes
Directory of L:\FOOBAR\Garmin Nuvi 265WT GPS
02/02/2010 02:13 AM <DIR> .
02/02/2010 02:13 AM <DIR> ..
12/13/2009 12:46 AM 32,881,087 Garmin nuvi 265WT - ComputerTV Review.mp4
12/13/2009 12:53 AM 135,552,201 Garmin Nuvi 265WT - during flight.mp4
12/13/2009 12:53 AM 8,810,775 Garmin nuvi 265WT - Features Tutorial.mp4
12/13/2009 01:03 AM 39,149,908 Garmin nuvi 265WT - HSN - 10-25-2009.mp4
12/13/2009 12:50 AM 8,299,497 Garmin Nuvi 265WT - Review.mp4
12/13/2009 12:56 AM 77,814,682 Garmin Nuvi 265WT - Traffic Reporting Part 1 .mp4
12/13/2009 12:51 AM 22,915,446 Garmin Nuvi 265WT - Traffic Reporting Part 2.mp4
7 File(s) 325,423,596 bytes
Directory of L:\FOOBAR\Microware
02/02/2010 02:13 AM <DIR> .
02/02/2010 02:13 AM <DIR> ..
01/08/2010 02:20 AM 1,625,272 Mark Hawkins at PennFest 2000.mp4
01/08/2010 02:20 AM 9,565,973 Tandy Color Computer Easter Egg.mp4
2 File(s) 11,191,245 bytes
Total Files Listed:
10 File(s) 356,065,489 bytes
8 Dir(s) 900,380,962,816 bytes free
Try the backup script again. This time nothing should copy since nothing changed.
C:\utils\nas>REM Copy from DLINK NAS to external hard drive 'L:'
C:\utils\nas>robocopy "\\dlink-59c1b6\Volume_1\Media\Informational" L:\FOOBAR /MIR /FFT /Z /LOG:test.log /TEE
Log File : C:\utils\nas\test.log
-------------------------------------------------------------------------------
ROBOCOPY :: Robust File Copy for Windows
-------------------------------------------------------------------------------
Started : Tue Feb 02 02:14:53 2010
Source : \\dlink-59c1b6\Volume_1\Media\Informational\
Dest : L:\FOOBAR\
Files : *.*
Options : *.* /FFT /TEE /S /E /COPY:DAT /PURGE /MIR /Z /R:1000000 /W:30
------------------------------------------------------------------------------
1 \\dlink-59c1b6\Volume_1\Media\Informational\
7 \\dlink-59c1b6\Volume_1\Media\Informational\Garmin Nuvi 265WT GPS\
2 \\dlink-59c1b6\Volume_1\Media\Informational\Microware\
------------------------------------------------------------------------------
Total Copied Skipped Mismatch FAILED Extras
Dirs : 3 0 3 0 0 0
Files : 10 0 10 0 0 0
Bytes : 339.57 m 0 339.57 m 0 0 0
Times : 0:00:00 0:00:00 0:00:00 0:00:00
Ended : Tue Feb 02 02:14:54 2010
We are good.