I've already done a How-To for upgrading code on Aruba OS-CX switches available here. However I thought it was worth a couple of lines about mounting a USB device to enable code installs when the switch is offline.
Once the USB device is inserted in the port we need to mount it as below:
6300# usb mount
We are then able to access the filesystem (FAT32) on the USB drive.
6300# show usb file-system /mnt/usb: ArubaOS-CX_6400-6300_10_07_1061.swi ArubaOS-CX_6400-6300_10_08_1040.swi 6300#
It is then very similar to the normal upgrade process, with the copy command referencing the usb: file location as below:
6300# copy usb:/ArubaOS-CX_6400-6300_10_08_1040.swi secondary
Followed by the appropriate boot system command as per the other guide here.