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There are a few threads here about the troubles trying to create a TRS-80 Single Sided disk from an image. Specifically, an image not done with the SCP. The general workflow when trying to create a disk with an image found online is:

1) Find image online
2) Use HxC software to convert it to .SCP format
3) Use SCP to write image to disk

The problem lies between #2 and #3. The HxC software does not properly convert the image, i.e. Byte $04 the Mfg/Disk_type is set to $14 when it should be $41. Byte 05, the #Revs is set to $03 when it should be $01. If this is set to anything other than $01 the SCP software ignores Byte $08, Bit 1 which tells the SCP software to use index mode. 

The SCP software also does not know what to do with a 40 track, single sided image and tries to write it as a 20 track, double sided image. You can use HxC to add a Side 1, fix the bytes explained above with a hex editor and the SCP software seems to be happy and write correctly but the disk does not work.

To work around this, I took the original image, converted it to an HFE format which will work on a Gotek. I was able to boot the TRS-80 Model 4P from the Gotek and then copy the image running on the Gotek to the floppy disk. Reconnecting the floppy drive as D0 I could boot from the floppy disk. I then imaged the disk with the SCP and wrote this image back to the floppy disk and was able to boot the machine.

The files I have created during this saga are:
TRDOS80.IMD - Original single sided image from web. For 5.25" DD media.
TRDOS80_IMD.hfe - Original image -> HFE w/HxC. Works on Gotek.
TRDOS80_IMD.scp - Original image -> SCP w/HxC. Writes as 20 track DS instead of 40 track SS.
TRDOS80_IMD_2S.scp - Original image -> SCP w/HxC, added side 1. Seems to write OK, does not work.
TRDOS80_IMD_wSCP.scp - Disk make w/Gotek imaged with SCP. Works when written back to disk.

Here is a link to the ZIP file which is too large to attach to post: https://1drv.ms/u/s!AtH4vpaZnzX7lcFFPNgp...g?e=Al8t4g
I am hoping that Jeff will address this issue with the HxC software.