So so sorry, I thought I had included it. It is a Latitude 5591. Thank you so much.

1. When you ask questions about compatibility with a specific laptop, you need to tell us the laptop model. Then we can look up information in Dell manuals and specifications. Since you did not tell us your model, I cannot do that.

2. If you currently have a 1 TB M.2 PCIe in that laptop, yes, you can easily switch to a 2 TB or even a 4 TB.

3. If your laptop has a 2.5" drive bay with connector/cable, then yes, you could install a 2.5" SATA drive. Your system drive should be the M.2, so the 2.5" drive should be for storage.

4. I cannot know if you can take your external drive to install internally, as again you did not tell us anything about it. If it is a standard 2.5" SATA drive in a separate USB interface or enclosure, then you could do that. If the external drive came with its own enclosure, such as for example a Western Digital Passport drive, then it may not be possible. Some of those models, such as the Passport drives, do not have a standard SATA connector on the drive. So even if you crack the case and remove the drive you would not be able to connect it inside your laptop.

No one can answer without the specific system model number (Latitude 7430, 5430, etc.).  Look on the powerup screen - F2 a few times at powerup -- to locate it.

So so sorry, I thought I had included it. It is a Latitude 5591. Thank you so much for your detailed answers tho. The external drive in question is a 1TB that was pulled from a laptop that came through our shop unrepairable. Thanks again

Storage options are listed below.  If your system came with only one M.2 drive, you may need additional hardware to mount a 2.5" SATA drive.

https://www.dell.com/support/manuals/en-us/latitude-15-5591-laptop/lati5591_setupandspecs/storage?guid=guid-fb2a6024-c60c-44e9-bc77-f2ca39838e79&lang=en-us