This was problem with my SP-404. First of all, 1GB are nowadays getting fairly rare already as it is. As even 4GB is small by todays standards.
But when I bought "thomann 1GB" flash, it didn't work on sampler. So luckily I had some friend with other old flash card he used in his camera, so that worked just fine.
I assume it's not only about size, but other things too, like speed of data which may cause issues. I had one card which "almost worked". Basically you could get sampler working, but it would randomly stop samples - both from flash and internal memory. As soon as it was taken away, internal memory worked just fine.
Good luck hunting suitable old flash....