What's the output you get from "usb start" ? Should be someting like
Marvell>> usb start
(Re)start USB...
USB: Active port: invalid port number 2, switching to port 0
Register 10011 NbrPorts 1
USB EHCI 1.00
scanning bus for devices... 4 USB Device(s) found
Waiting for storage device(s) to settle before scanning...
scanning bus for storage devices... Device NOT ready
Request Sense returned 02 3A 00
2 Storage Device(s) found
Marvell>>
Does it see your sd card as device 1? i.e.
Marvell>> usb storage
Device 0: Vendor: Generic Rev: 9451 Prod: STORAGE DEVICE
Type: Removable Hard Disk
Capacity: not available
Device 1: Vendor: Generic Rev: 9451 Prod: STORAGE DEVICE
Type: Removable Hard Disk
Capacity: 15407.0 MB = 15.0 GB (31553536 x 512)