I could not isolate the issue, but here are the steps that I took:
Instead of starting normally, an "Error" popup window appears. Unfortunately, I have Russian locale installed, so the text might not match exactly since I've translated it manually. The title is "ImageGlass.exe - Disk missing", the text is "Device does not contain a disk. Insert a disk into device D:." Three buttons exist: "Cancel", "Repeat" and "Continue"; however, pressing any button or closing the popup just brings it up again.
The application should start normally.

A disk drive with designated letter D: exists in my system but has not been used in a while. My OS is installed on an external hard disk.
This issue has been reported by me previously on the ImageGlass Community Google Group: https://groups.google.com/d/msg/imageglass/IU4nqL3k_5U/vlrRkTQsAQAJ. The issue has appeared again. Unlike my previous case, the title now mentions ImageGlass.exe, not igcmd.exe, however.
A similar issue is #360, but I have not accessed my disk drive since last several successful launches of ImageGlass.
Drat. I've been banging my head on this one periodically. I had hoped it was under control. I will look again. My apologies for the hassle.
When my system gets into this state, the only thing I've found that will work around it is to insert something so drive D: is active again. Possibly "fixable" by dismounting drive D:.
I still need to identify and fix the root cause, but the following solves the problem for me:
(from here)
Phap - I have something for you to investigate, related to this issue.
When I scan the 7.5.1.1 release imageglass.exe for strings, the following string appears:
D:\DEV\ImageGlass\Source\ImageGlass\obj\x64\Release\ImageGlass.pdb
When I build from Github (release or debug), said string does not appear. I also find that
string in the last Moon release.
It looks like a debug file reference; I don't know what the "D:" drive is on your system.
Could you check your "publish" build environment for some sort of setting which might
introduce this reference into the EXE?
[To find strings in binaries, I use [strings](https://docs.microsoft.com/en-us/sysinternals/downloads/strings)]
Release builds have "debug info" set to "pdbonly". Should be "none".
Should be fixed in v7.6