Spinal are you using External SL IV?
Because i could never get ext sl iv to work on windows 7 x64.I even bought another one to see if my old where "faulty".
Also VMWare on windows 7 x64 will not recognize the usb device(!) (Note:VMWare on windows 7 x64 will not recognize any device not just SL IV).
Your only solution is windows xp 32bit or linux +vmware with windows xp 32bit installed and then external sl iv will work great.
Of course SNES/SFC myth cart works great on windows 7 x64.Just like any cart from the MYTH series.
Hope it helps! 
thats not fully true. I managed it befor the neo myth cart just with a SL IV as follow:
installed the latest Neo Myth Ultra software
plugged in the SL to install the 64bit driver
de-installed the Neo Myth Ultra Software (from add/remove programms) and deleted in c:\NEO2 Ultra Menu folder
installed the original 2.50 Neo2 software
installed (as upgrade) the NeoMyth MD 2.83 software
after those steps I was able to use the external SL IV with Windows 7/64bit for Neo2 1024 cart however I was not able to use it anymore after upgrade to 2.99.
since the Neo Myth Cart has a build in SL IV and its compatibilty with 2.99 I do not care anymore for the external SL
