Does anybody know how long installing El Capitan onto a USB 2 pendrive from another USB 2 pendrive takes on a MacBook Air? I'm trying to resucitate a discarded 2013 11″ MacBook Air with no internal drive and hence no OS.

The Apple Store Genius bar tested it and found the only problems to be the missing drive and broken display backlight.

Since I don't know if it's worth spending money on I'm doing an OS install to test it. We got Safari working with Wi-Fi from the installer so we know it does run.

But I only had USB 2.0 pen drives. We have the OS installer on an 8 gig one and we are installing to a 16 gig one.

The progress bar moved very slowly and the estimated "time remaining" was hilariously wrong. The progress bar eventually reached 100% and the time estimation went away but the USB activity light is still flickering with activity.

I know USB 2.0 is very slow, especially for writing and especially for a large amount of data. The install has been going for about 43½ hours so far with no error messages but also no other clues as to whether it's still going OK apart from the USB activity light.

I know this is an unusual situation and there are faster ways to do it but require spending money before knowing how well the machine will work.

Does anybody know or can find information from somebody who's done this before about how long it ought to take?
I've never done this before, but from the looks of it, the speeds will be limited by how fast your thumbdrives go.

If I remember correctly, an El Capitan install is around 8-10 GB in size, so the 16 GB one you're installing to should be sufficient.

USB 2.0 in reality can IO data at around 30-40 MB/s (although this depends on the make and model of your pendrive).

Using these factors we can calculate that an installation can take up to 6 hours for copying alone, not factoring the CPU overhead in installation.

Although 72 hours is a bit of a stretch, as long as your USBs remain flashing I would be fairly certain the system is doing something and not worry about it halting.