Booting MacMini (late 2014) from external ssd

You should be able to start the mac from another drive or another partition

within the internal drive by selecting one; there may be more than one way

that could work to accomplish this action. Note "Startup Disk Preference"..

• How to select a different startup disk - Apple Support

https://support.apple.com/en-us/HT202796

You should be able to use the Option key on startup to select a bootable

volume either on external drive (if correctly prepared) or internal partition

or maybe a network volume with macOS or suitable working system.

Read this ^ link carefully because it covers the main ways in modern mac:

it has a good chance of working in your case. (If installation was correct &

is seen by the mac hardware.) ~ Another look at similar information shows

an older page with just this feature; where a menu of options can appear:

• How to Change the Mac Startup Drive on Boot - How to??

http://solution2everything.blogspot.com/2013/07/how-to-change-mac-startup-drive-on-boot.html

And sometimes issues may result after an upgrade or change in the system

..so the older method fails to function; sometimes the OS X doesn't like to

'dual-boot' so you may have to forage to determine what else may work..

Some keyboard shortcuts may force the Mac to try & find a bootable volume

while a newer macOS and/or file system may balk when using older method.

Maybe the external SSD is slow to wake up? Does it has its own power supply or is it powered via USB?

The missing startup bong in new Macs makes timing Option-boot difficult: press Option a little too early or a little to late boots normally and requires another try (with practice I now have about 80% success). If I want to to be sure, I shutdown and startup pressing Option (I usually don't want to visit the Startup Disk GUI).

There is a Terminal command to set the boot disk but I don't know if there is any practical difference to the GUI regarding your problem:

https://www.techwalla.com/articles/how-to-change-the-mac-startup-disk-from-the-command-line

FYI, some external enclosures, docks, and adapters may not boot a computer (especially a Mac). I always stick with just a few name brands such as Plugable, OWC, and StarTech. Of course some other brands work too, but these are the ones I'm familiar with and like. Years ago we purchased a drive enclosure which wouldn't work with a Mac at all, but worked fine on a standard PC. Macs can be bit particular about their boot drives sometimes.

What brand of drive, enclosure/dock/adapter are you using?

You can also try connecting the drive to a powered USB3 hub with UASP support. Use a well respected name brand product as the quality of many hubs is substandard and will just cause problems.

For what it is worth, I've seen some bus powered drives that cannot boot a computer from a cold start (turning on the power) but can from a warm start (rebooting). I'm sitting in front of an iMac that is booting with an external SSD which could boot my mini without a power brick but not the iMac I'm currently using. Not sure why that's the case, but there you go. Normally I'd not even try using a bus powered drive to boot up but I was down to my last outlet and didn't have anything I could unplug!

Cupra1979 wrote:

cuts 2-3 cm of screen on the four sides

Check the Underscan slider in Displays system preference.