Can't upgrade or change firmware on WNR3500Lv2 already running DD-WRT

Hello all, I dusted off my old WNR3500Lv2 today, its actually running "DD-WRT v24-sp2 (03/25/13) vpn - build 21061" already and all is working well but i was trying to switch over to Tomato just to try it out. It starts doing the firmware upgrade, says expect to wait 300 seconds but afer 10 seconds says successful, please wait and then boots up again still in DD-WRT.

I've since tried flashing newer versions of DD-WRT with the same result.

Have tried....

tomato-Netgear-3500Lv2-K26USB-1.28.AT-RT-N5x--3.3-138-AIO.bin (changd to .bin after original name .chk didn't work)

tomato-Netgear-3500Lv2-K26USB-1.28.AT-RT-N5x--3.3-138-VPN.chk

and dd-wrt.v24-K3_WNR3500Lv2.bin

But everytime, it says successful and then reboots into the same original firmware.

Any ideas what I'm doing wrong?

Thanks