From mid 2012, manufacturers of USB flash drives like SanDisk buckled to pressure from Microsoft, and changed the structure of all of their USB flash drives so they are no longer seen by Windows as “Removable Media”. This was in order to meet with Windows 8 certification. At the same time, they announced that they no longer supported being able to boot from these drives.

for more details see http://kb.sandisk.com/app/answers/detail/a_id/12830

This issue is not confined to Avast, it also affects other rescue or recovery software that is based on booting to a USB flash drive.

It is possible to make a workable Avast bootable rescue USB flash drive from the option provided in Avast 2014, but only on flash drives that are seen as “removable”.

Some software specifically looks for “removable media” and doesn’t show the newer “non removable” flash drives as an available destination at the start of the process.

But even when a “non removable” USB flash drive can be selected as a destination, and the make rescue media process is completed, some PC’s see the “non removable” flash drives as normal hard drives in the boot up selection screen, but cannot boot from them, and this applies even if you make a drive image from a “Removable” rescue drive that does work, and restore it to one of the “non removable” flash drives.

Edit: There is a difference in the controller chips in the “Non removable” flash drives. There are reports of some success toggling the removable bit in the controller, but only on specific model drives. On other brands and models it has either not worked, or has resulted in the complete failure of the drive.