I also have purchased the D-link DSL-2750u last week. Afterwards i found out there is an USB port. Tried my seagate 250 GB, working ok, but the transfer rate is around only 2 MBps even when connected by the 10/100 LAN port. Then i updated the firmware from D-link website Code:
ftp://support.dlink.co.in/broadband/DSL-2750U/firmware/DSL-2750U_IN_1.10_B038_K8B_0K-IN-00-2916_20110822.zip
but the speed is still the same. Otherwise everything is OK.
(for firmware update, unzip the archive, you'll get an .img file. Connect the device with wired LAN only, don't try update with WiFi. Access modem by typing 192.168.1.1 in browsers address bar, username and password both are 'admin'. Go to Maintenance > Firmware update > browse the .img file and click update. WARNING - updating takes about 4-5 minutes, don't be impatient, don't turn off or power down the device. If you do so, the device will be worth the same as a brick and the warranty will be void)

Originally Posted by
liberal
Have you found the solution for the update not working..., also tried to download product manual from Dlink india website .....
.....
.....
..... for the CD.
I think Dlink website is not managed well, and the firmware image file may itself be corrupt.
I am struggling to get my Seagate 1Tb 3.5" external USB working with this modem router.
I have found and updated the firmware from the very link you provided.
Go to support > Firmware
And all my storage device is working with it (although the transfer speed is very slow). Just followed the instruction above by 'essbebe'
I am still searching for a solution, but it seems to me that the hardware on the modem-router is itself slow.