While creating the OS layer in AppLayering 4.x we get the error message:
An error occurred. The required file 'OSType.txt' was not found on your OS disk. Please obtain the application layering OS machine tools, run 'Setup_x64.exe' on your OS image, and then try creating the OS layer again. 

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AppLayering only supports MBR and Hyper-V Generation 1 machines. It does not supports GPT and Generation 2 machines.

Capture ELM logs and in camlogfile we could see, if GPT is being used.

2017-11-17 11:43:46,085 INFO Threadpool worker Shell: EXEC:         Device Boot      Start         End      Blocks   Id  System
2017-11-17 11:43:46,085 INFO Threadpool worker Shell: EXEC: /dev/nbd1279p1               1  4294967295  2147483647+  ee  GPT

Note: App Layering will only look at the first partition or two, so if we are running setup64.exe and it's saving on a 3rd or 4th partition, it'll never look at that partition by design, so it will never find ostype.txt there.

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Problem Cause

GPT and Gen2 VMs are not supported.