Hard drives are slower at writing than reading, in general. But I've not personally noticed such a big offset. It could be a drive or enclosure limitation, but you've not specified any make/model info. 280 Mbps is a good USB2 read speed, especially when pulled over a wifi connection.
It may not be the fault of the wifi. Have you considered it could be the USB interface (the drive enclosure) or the Odroid itself? Have you done any testing on that side?
How is the USB HDD being powered, from the Odroid usb ports or external AC adapter. If not external AC, maybe the Odroid can't keep up the power.
Consider testing:
- with your laptop connected via ethernet to your home router (hopefully it has an ethernet connection) to eliminate typical wireless issues.
- read/write speeds of your USB HDD on your laptop, as a means to compare relative speeds and eliminate possibilities.
- uploading from your laptop to some other network device's internal storage (not usb attached) if possible to see if the speed discrepancy follows (would help to eliminate the Odroid itself).
- with a USB thumb drive instead of the USB HDD.
Good luck.