Since about E2B v1.73, if the second partition is a small hidden type 21h partition that is placed there by RMPartUSB when you use RMPrepUSB or the MAKE_E2B_USB_DRIVE.cmd script, this partition is not removed when you switch to a .imgPTN file - this is only done if the extension is exactly ".imgPTN" or ".imgPTNAUTO".
This was done because some Legacy BIOSes would recognise the USB drive as a floppy disk and thus fail to boot grub4dos and boot to the CSM menu unless a 2nd partition was present (e.g. some EeePCs).
This was done because some Legacy BIOSes would recognise the USB drive as a floppy disk and thus fail to boot grub4dos and boot to the CSM menu unless a 2nd partition was present (e.g. some EeePCs).