Most of my photos are on an external hard drive. I have tried importing them, am able to select a folder but then the programme defaults to 'no external device selected'. I have also tried importing a folder - the photos then appear temporarily in the database but as soon as I click on one, they all disappear.
as are ALL of mine and they show in PM without problem! So, having photos on an external drive is not, per se, causing this difficulty for you.
First, have you allowed PM long enough to actually create its database of your photos? Depending how many you have, it can take a long time!
However, since it seems that the problem might be that PM loses its connection to your external drive, how, exactly, is it connected to your computer? USB, USB2, USB3? I rather think that Windows is turning the drive "off" if it believes it is not being used, especially if your computer is a laptop.
To check this do the following: in the Search Box (left hand end of the taskbar) type cpl. On the screen that opens, choose "Control Panel App". Depending on your settings, you should then see a screen which, towards the top right, has an option to "View by...". From the dropdown choose "Large icons". On the next screen click on "Power Options". Whatever setting is selected here, choose the option to "Change plan settings". On the next screen, choose "Change Advanced Power Settings".
That will open a screen that looks like this:
Note; this is on my computer, yours is likely to look at least slightly different.
Find the item I have drawn the box around and make sure the highlighted item says "Disabled".
Close all these various screens you have opened and try again to have PM create its database.
There is one potentially serious downside to these changes (assuming they work for you); if you are using a laptop and your external drive is drawing its power from that laptop, your battery will empty rather more quickly than previously because your external drive will always be "on".
Good to hear you've resolved the issue and thanks for the feedback.
Unfortunately, if I am right about the power settings causing the problem, unplugging and replugging will be only a short-term solution because eventually, Windows will decide again to turn the drive off!