diff --git a/e2e-tests/fixtures/ExternalSources.ts b/e2e-tests/fixtures/ExternalSources.ts
index ec04ddf63b..083fc26441 100644
--- a/e2e-tests/fixtures/ExternalSources.ts
+++ b/e2e-tests/fixtures/ExternalSources.ts
@@ -4,6 +4,7 @@ export class ExternalSources {
   alertError: Locator;
   closeButton: Locator;
   createTypesButton: Locator;
+  createTypesModal: Locator;
   deleteSourceButton: Locator;
   deleteSourceButtonConfirmation: Locator;
   derivationATypeName: string = 'DerivationA';
@@ -67,6 +68,7 @@ export class ExternalSources {
 
   async createType(typeName: string, typeSchema: string, isSourceType: boolean) {
     await this.createTypesButton.click();
+    await expect(this.createTypesModal).toBeVisible();
     if (isSourceType) {
       await this.page.getByRole('radio', { name: 'External Source Type' }).click();
     }
@@ -80,8 +82,8 @@ export class ExternalSources {
     } else {
       await this.waitForToast('External Event Type Created Successfully');
     }
+    await expect(this.createTypesModal).not.toBeVisible();
   }
-
   async deleteDerivationGroup(derivationGroupName: string) {
     await this.page.getByRole('button', { exact: true, name: 'Derivation Group' }).click();
     await this.page.getByRole('row', { name: derivationGroupName }).hover();
@@ -195,6 +197,7 @@ export class ExternalSources {
     this.externalSourcesTable = page.locator('#external-sources-table');
     this.createTypesButton = page.getByLabel('Create external source types or external event types.');
     this.manageGroupsAndTypesButton = page.getByLabel('Manage and inspect existing');
+    this.createTypesModal = page.locator(`.modal:has-text("Create New External Source/Event Types")`);
   }
 
   async uploadExternalSource(
@@ -202,6 +205,9 @@ export class ExternalSources {
     inputFileName: string = this.externalSourceFileName,
   ) {
     await this.fillInputFile(inputFilePath);
+    // Wait for all errors to disappear, assuming stores are just taking time to load
+    await this.page.getByLabel('please create one before uploading an external source').waitFor({ state: 'hidden' });
+    await this.page.getByLabel('Please create it!').waitFor({ state: 'hidden' });
     await this.uploadButton.click();
     await this.waitForToast('External Source Created Successfully');
     await expect(this.externalSourcesTable).toBeVisible();
diff --git a/e2e-tests/tests/external-sources.test.ts b/e2e-tests/tests/external-sources.test.ts
index 4cd12d3464..1efcd0c4ce 100644
--- a/e2e-tests/tests/external-sources.test.ts
+++ b/e2e-tests/tests/external-sources.test.ts
@@ -116,7 +116,6 @@ test.describe.serial('External Source Error Handling', () => {
       externalSources.externalSourceTypeSchema,
       true,
     );
-    await page.waitForTimeout(5000);
     await externalSources.uploadExternalSource();
     await externalSources.deselectSourceButton.click();
     await expect(externalSources.externalSourcesTable).toBeVisible();
diff --git a/src/components/external-source/ExternalSourceManager.svelte b/src/components/external-source/ExternalSourceManager.svelte
index c4aa77efe5..2149813000 100644
--- a/src/components/external-source/ExternalSourceManager.svelte
+++ b/src/components/external-source/ExternalSourceManager.svelte
@@ -7,6 +7,7 @@
   import ExternalEventIcon from '../../assets/external-event-box-with-arrow.svg?component';
   import ExternalSourceIcon from '../../assets/external-source-box.svg?component';
   import { catchError } from '../../stores/errors';
+  import { externalEventTypes } from '../../stores/external-event';
   import {
     createDerivationGroupError,
     createExternalSourceError,
@@ -157,6 +158,8 @@
   let file: File | undefined;
   let externalSourceFileInput: HTMLInputElement;
   let parsedExternalSource: ExternalSourceJson | undefined;
+  let isUploadDisabled: boolean = true;
+  let uploadDisabledMessage: string | null = null;
 
   // For filtering purposes (modelled after TimelineEditorLayerFilter):
   let filterExpression: string = '';
@@ -336,6 +339,20 @@
     return planDerivationGroupLink.derivation_group_name === selectedSource?.derivation_group_name;
   });
 
+  $: if (parsedExternalSource !== undefined) {
+    if ($externalEventTypes.length === 0) {
+      uploadDisabledMessage = `No external event types are currently defined, please create one before uploading an external source!`;
+      isUploadDisabled = true;
+    } else if ($externalSourceTypes.length === 0) {
+      uploadDisabledMessage = `External Source Type "${parsedExternalSource.source.source_type}" is not defined. Please create it!`;
+      isUploadDisabled = true;
+    }
+    isUploadDisabled = doesSourceTypeAndEventTypesExist(parsedExternalSource);
+  } else {
+    isUploadDisabled = true;
+    uploadDisabledMessage = null;
+  }
+
   // Permissions
   $: hasCreatePermission = featurePermissions.externalSource.canCreate(user);
 
@@ -463,6 +480,27 @@
       return featurePermissions.externalSource.canDelete(user, [externalSource]);
     }
   }
+
+  function doesSourceTypeAndEventTypesExist(externalSource: ExternalSourceJson): boolean {
+    // Check that the External Source Type for the source to-be-uploaded exists
+    const externalSourceType = $externalSourceTypes.find(sourceType => sourceType.name === externalSource.source.source_type);
+    if (externalSourceType === undefined) {
+      uploadDisabledMessage = `External Source Type "${externalSource.source.source_type}" is not defined. Please create it!`
+      return true;
+    }
+
+    // Check that all the External Event Types for the source to-be-uploaded exist
+    const newSourceExternalEventTypes: string[] = Array.from(new Set(externalSource.events.map(event => event.event_type)));
+    for (const eventType of newSourceExternalEventTypes) {
+      if ($externalEventTypes.find(eventTypeFromDB => eventTypeFromDB.name === eventType) === undefined) {
+        uploadDisabledMessage = `External Event Type "${eventType}" is not defined. Please create it!`
+        return true;
+      }
+    }
+
+    uploadDisabledMessage = null;
+    return false;
+  }
 </script>
 
 <CssGrid columns={uiColumnSize}>
@@ -636,6 +674,7 @@
         >
           <AlertError class="m-2" error={$createExternalSourceError} />
           <AlertError class="m-2" error={$parsingError} />
+          <AlertError class="m-2" error={uploadDisabledMessage} />
           <div class="file-upload-field">
             <fieldset style:flex={1}>
               <label for="file">Source File</label>
@@ -660,7 +699,7 @@
                 </div>
               {/if}
               <button
-                disabled={parsedExternalSource === undefined}
+                disabled={isUploadDisabled}
                 class="st-button w-100"
                 type="submit"
                 use:permissionHandler={{