Q: iPhone or iPad sometimes "dismount" from Photos and will not mount again

Hello all,

Weekends I normally have Photos app on and both my iPhone 6s plus and iPad mini 4 will be seen mount on it on the left column. (on my iMac)

Sometimes intermittently I would notice one of them would "disappear" from the left hand side column under the 'Import' tab, e.g. either the iPad mini 4 or the iPhone, I remove/unplug the device, plug in again, but it fails to mount, i.e. if I have new photos to import from the iOS device (and choose to delete from the device) I can't do it.

I kind of know at least the USB cables and the mounting "works" because I can see both devices in iTunes.

I've tried:

1. Off and On the iOS device
2. Plug/unplug USB cable, even change port behind iMac
3. Quit & relaunch Photos app

All above failed to work.

The only working way to have see both devices again seem to be rebooting the iMac. That's not something I want to do often.

Thanks.

P.S. Photo show only my iMac mini 4 in Photos app, but my iPhone 6s plus is also mounted. I can see them both in iTunes.