extern
external
Coming from outside, located outside a particular area, system, or organization.
Für das Projekt haben wir externe Berater engagiert.
We hired external consultants for the project.

external
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external
Coming from outside, located outside a particular area, system, or organization.
Für das Projekt haben wir externe Berater engagiert.
We hired external consultants for the project.

'Extern' (external) can stand as an adjective before a noun and is then declined. But it can also stand unchanged after a noun or verb and then functions like an adverb.
Wir brauchen eine externe Perspektive auf das Problem.
We need an external perspective on the problem.
In a company or organizational context, 'extern' (external) is often combined with people or services. Typical examples are: 'externe Berater' (external consultants), 'externe Mitarbeiter' (external employees), or 'eine externe Prüfung' (an external audit/examination).
Die Firma beauftragte eine externe Agentur für das Marketing.
The company hired an external agency for the marketing.
In the technical field, 'extern' (external) describes devices that are connected to a computer. Common combinations are: 'externe Festplatte' (external hard drive), 'externes Laufwerk' (external drive), or 'externer Monitor' (external monitor).
Du solltest deine Fotos auf einer externen Festplatte sichern.
You should back up your photos on an external hard drive.
The meaning of 'extern' (external) becomes clear from the context. If people are talking about companies, administration, or education, it usually means 'from outside the organization.' If people are talking about computers, it refers to a connected device.
Die Universität lässt die Prüfungen extern korrigieren.
The university has the exams graded externally.
outer / external
'äußer' and 'extern' both have to do with 'outside', but they are used differently. 'äußer' describes the outer side, the outer area, or the visible appearance of something. It is almost always used with an appropriate adjective ending, for example 'äußere Wand' (outer wall) or 'äußeres Zeichen' (outward sign). 'extern' means that something comes from outside a system, organization, or device, or is located outside it. It often fits in work, school, technology, or administration, for example 'externe Berater' (external consultants) or 'externe Festplatte' (external hard drive). Learners may confuse the words because both form an opposite to 'inside'. For a visible surface or edge, 'äußer' fits; for something outside a group, company, or device, 'extern' fits.

Für das Projekt arbeitet die Firma mit einem externen Experten zusammen.
For the project, the company is working with an external expert.
Die äußere Tür des Hauses ist wegen des Wetters stark beschädigt.
The outer door of the house is badly damaged because of the weather.