progress bar stopped working w/USB

[procount]

(BUG)
Whilst testing my fixes for partition labels in a newly built NOOBS from the latest commit, I booted NOOBS from the SD card and was installing LibreELEC and another small OS from a USB stick to another USB stick on a Pi2, so the boot partitions would still be on the SD card but the rootfs would be on the USB Stick.

When it was installing them, I noticed that the progressbar on the slide show did not move - it stayed at 0% complete, and yet the installation went ahead ok. It's just that the feedback of progress was not updating.

When I removed the target USB stick and installed from USB to SD card, it worked fine.

So I downloaded the latest NOOBS_lite 2.4 from the web and tried with that to test if my changes had caused the problem but the same thing happened. I will investigate a little further whilst testing my fixes and report if I find the cause.

[procount]

It's because it is not accounting for any writes to the _bootdrive when _multiDrives is true. It's quite noticeable when installing LibreELEC because all the image is in the boot drive and the rootfs is empty.

[procount]

Fixed. I'll submit a PR in due course.