To be brief, check your ISO for \sources\install.esd if:
Whilst using setup.exe from your install media from within WIndows.
If so, you can convert from ESD to WIM using:
Windows 10 TP has both WIM and ESD install.xxx files for their installation. For example my Build 9841 had install.wim the later build, Windows10_TechnicalPreview_x64_EN-GB_10041 has install.esd. This change may be the reason behind some fails.
Not all ESD files can be Decrypted by decrypt.cmd tool. This maybe becuase the ESD doesn't contain a Boot Image, just files. Possibly, I'm not sure. So some ESD updates that have failed to decrypt to ISO may not be Boot type.
The fails may not be limited to running install from within a previous build or version of Windows.
For info. I'm testing from Windows 8.1 after having a good run of updates from 9841 to 10041 and installing clean with 10049. HP DV7 hardware no issues.
So if having boot fails using Boot Media, just check against ealier successful boots if WIM or ESD and note if your PC is BIOS or UEFI and if UEFI whether, MBR or GPT.
After conversion from ESD to WIM install completed from Windows 8.1 to 10049.
WiFi off as to ensure install was locally sourced.
No other files were altered. Setup accessed install.wim automatically regardless of extension and file format change.
ESD file with boot:
This ESD file show in the above ESD<>WIM converter shows 4 indexes. This ESD file will create a bootable ISO.
ESD file without boot:
With only 1 index, this ESD file will not create a bootable ISO. It will also create an error if used with decrypt
Here, the install.esd file only had 1 index and couldn't create a bootable ISO.
Meaning, not all install,esd contain boot information.
Other install.xxx locations include:
As always, testing may result in data loss - So backup everything you don't want to loose.