You can verify the problem when using the Disk Manager cartridge and do a disk check. It will fail after half of the tracks.
@Micko: It is not a matter of the track translate function - the problem is that the TI disk controllers try to handle drive/media track count mismatches. Specifically, if the drive can handle 80 tracks and you insert a 40 track disk, the controller ROM tries to detect that and will apply double physical steps for each logical track step. This is a behaviour from the TI ROMs, not from the MESS emulator.
Version 0.98 MESS introduced code on the assumption that one can safely set the media track count equal to the drive track count - which instantly broke the whole TI emulation (until v0.123 where I fixed it). My solution was to include a check in mflopimg.c, internal_floppy_device_load (look for DEVINFO_INT_KEEP_DRIVE_GEOMETRY). Have you moved that somewhere else?
I am aware that a much better solution would be to create a specific drive/image implementation instead of these tricks using basicdsk, and I already planned to do some work there, but for the time being, please make sure that the drive emulation and the medium emulation allow for different track counts. This is simply mirroring the real situation.