I used to have a USB virtual CDROM drive made by Zalman. It was very handy, it held a bunch of ISO files and you could select one to to present as a physical CDROM drive to a PC you are repairing or re-installing. I had trouble with it getting corrupted and it finally died. I missed it, but it died too quick to throw another $40 at one. The corruption left me in the lurch a couple times, and there's no easy way to recover from that in the field being that it's a 2.5" SATA drive in it. I also didn't like the jog wheel and the cumbersome method for adding ISO files. 

I was fixing a PC recently and had to make like 6 different Rufus images before the damn thing finally saw the drive so I could install. I looked again for the Zalman thing, and saw one a guy made from a raspberry pi. I built it, but it didn't work & had the files on another partition on the pi's SD. And another very cumbersome method for mounting the USB to add files that also seemed prone to corruption.  

I set out to make my own. I added a 2nd MicroSD and a cheap OLED screen to a pi zero, and got it working pretty quickly. Added 2 buttons and now I'm making a "hat". Then I'm making a case, with some additional MicroSD storage. 

The CDROM images are presented READ ONLY. When the MicroSD is presented it is NOT mounted on the pi. I suspect this is not the case with the Zalman, and it's not the case with the other pi version. I believe this one is less prone to corruption, and most certainly easier to recover should any occur.