Hi there!
When you install new software, its default installation location will be the startup disk you're currently using, which sounds like it's the external SSD.
While you cannot disable the internal disk, you can delete all volumes on it, so it doesn't appear in macOS apps, such as the Finder. Before doing so, check that there's no important data on it:
- Go to Finder, and choose Go > Computer, and choose the internal disk.
- Using Finder, browse the internal disk, and be sure that there's no important data on it. If there is, copy and paste it to a new location on your Mac: How to copy and paste on Mac - Apple Support.
When you're ready to erase the internal disk, you can follow the Apple Support article Erase and reformat a storage device in Disk Utility on Mac - Apple Support:
1. Open the Disk Utility app on your Mac by double-clicking Disk Utility in the /Applications/Utilities folder.
2. Choose View > Show All Devices.
3. In the sidebar, select the storage device you want to erase (the internal disk will be non-indented and at the top of hierarchy), then click the Erase button .
4. Click the Scheme pop-up menu, then choose GUID Partition Map.
If the Scheme pop-up menu isn’t available, make sure you selected the storage device you want to erase—it’s at the top of the hierarchy.
5. Click the Format pop-up menu, then choose APFS.
6. Enter a name you'll remember, such as "Delete".
7. Click Erase, then click Done.
After erasing the internal disk, you'll see a new APFS volume on it. Choose it, then select Edit > Delete APFS Volume... to remove the internal volume on your Mac.
Hope this helps! If you have any questions, let us know!
- Jack