From what I've noticed, if the drive in question is detected to have at least 15GB capacity then there's no problems. Any less (even 14.9GB) and the recovery image creation app will not allow you to use the drive. This may not be the only trigger for this issue but it's the only clue I've seen, so far.
Anyway, you'll want to follow Acer's instructions for using USB recovery drive patch to fix this problem. Although this specific case isn't mentioned in the document I've found that it does indeed fix the problem.
I've recreated the instrutions here for easy reference.
- Download the USB recovery drive patch.
- Extract the contents of the file to the desktop.
- Ensure the system is connected to AC power.
- Run the file PBRFix.exe from the desktop and click Fix.
- When the process is complete, you will receive the message, "Your recovery image has been fixed". Click OK.