Great question RHill! I'm always happy to help a fellow GPxTrack convert
No, it doesn't matter which slot you use, GPxTrack can handle it, as long as there is only 1 track in the folder,
like this. So only 1x .dat, 1x .wad, 1x .ini etc.
The important point to remember is - regardless of which slot - I do advise that the .wad file should have an original 2001 filename, e.g. nurburgring2001.wad or indianapolis2001.wad. But it doesn't matter which one. And you are free to rename all the other files (.dat, .ini, .gpi, etc) but the .wad is sensitive and needs to keep its original 2001 name.
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Intel NUC 8i3, 8GB RAM, MS Sidewinder Wheel