Hi, my first participation in this forum I've been checking a lot lately.
I actually run through the same issue of merging 3 recovery discs of a gateway laptop deprived from CD drive, I have burned those the RECOVERY DVDs with virtual CD as iso images ( the recovery crating software DOESN't allow Dual layer DVD! size limitation of each DVD 4.7 GB) which I transfered with a usb stick to my desktop and then burned each one of them on a separate USB stick for recovery purpose.
Now when you insert the first usb stick in the laptop in a recovery process, it is recognized and boots up fine and recovery process starts, but it will ask for the 2ND recovery DVD and doesn't recognize the 2nd USB stick ( that has the 2nd recovery CD image on it).
After multiple hours of search on this matter, I decided to try IROD8 trick, which is to gather the 3 DVDs manually to create one iso image on the USB stick (couldn't find any software that does that)
I used ultra iso (to keep boot image valid) to open RECOVERY1 DVD (and then mounted the RECOVERY2 and RECOVERY3 DVDs on virtual drives using magic iso or different software : compared files and folder METICULOUSLY
- deleted any referrers to CD2 and CD3
- gathered all recovery images from DVD2 and 3 in the DVD 1 images folder
- added patch folder present in 2 and 3 to dvd1 and gathered all files and folders from 2 and 3 in it
Burned the iso image obtained directly to usb 16 GB = TOTAL SUCCEsS , recovered laptop WHOLE disk structure with recovery partition and system partition which was amazing by the way.
Later I have tweaked the iso image after saving it, deleting the extras ( located in patch folder) in a way I got an iso image little enough to fit a DVD dual layer to be saved for later ( to free the usb drive and eventually will build an external slot loading slim dvd drive from ebay ! to use with the laptop)