Can I use the exact same Sims 3 folder if I reinstall the game?

An external drive as in it's outside of your computer and the connection is cabled to it externally?

Either one should be possible, but the game should never really be installed on an external drive. This is not typically a path to success. While it's true that an SSD will have faster read/write times, those only affect startup/save and catalog building time in CAS and Build/Buy, it doesn't affect actual gameplay as most of the game is loaded into and runs from RAM as we play it.

Some games, like TS4 as it so happens, get no benefit from running off of an SSD.

The issue is not read/write disc speeds, it's that no matter how good that external connection is, it will never be as good as an internal drive can have and the game will tend to not react well to all of the resulting delays in data transfer. External drives are really meant for file storage and retrieval, you can certainly run stored videos that way since they tend to buffer a lot, but they are not meant for complicated program installs.

Is this the laptop that already had two internal drives? Seems to me that the internal D drive could have been replaced with an SSD without having to disturb the operating system, as that would be be installed on the C drive.

I meant the Samsung T5 SSD, so it connects through USB C. (I wanted one with TB3 but that's waaaaay too expensive for my wallet) It's advertised as something that's good for running games from it so that's why I bought it instead of switching the SSD (I want to have an HDD for storing data as I'm really paranoid about shortening the SSDs lifespan by constantly downloading and deleting big files on it even though I know it takes a really long time to achieve that).
Do you know if I'll have to reinstall all my store content again maybe? Or can I just copy paste the package files that were created when installing them?

Afraid my prediction remains that this will not work well. I am sure that some games at some level can run off of/across a USB C connection, but not one that constantly seeks program data and caches throughout play like TS3 does.

The concern about SSDs having short lives due to overwrites was valid with very early models of the beasts, I don't think that's really an issue anymore.

No, you do not have to redownload store or any other content when switching the hard drive on which Documents and the TS3 user game folder reside. Store content that is already installed lives in the DCCache Folder plus InstalledWorlds where relevant, and Library for actual venues. Those folders should seamlessly copy over with everything else.

@Great_Ulcer9999 - Hi there. We have a thread set up on this board for those with New Member status who require assistance but cannot yet start up their own threads. That would be a good place to continue this conversation in case this one gets closed down by the moderators for being a bit stale.
https://forums.thesims.com/en_US/discussion/893122/thread-for-new-members-to-post-their-sims-3-game-issues#latest

Did you ever have the game and its EPs installed on the default drive on this system? If so, and you have since uninstalled them, a Registry cleanup would be in order to get rid of Windows Registry entries left behind that indicate the game is still installed on C or wherever the default is. The free version of Piriform's CCleaner is a good tool to use if this step is required.

Afraid I can't say why the disc installers would be telling you that the game is already installed. But we do know that selecting the root level D : as a custom install destination will cause issues. There needs to be a subfolder created on D first, such as D :\Program Files (or whatever you want to call it) that the installers should be directed to in case this is part of the issue.

Hmmm I managed to pick the SSD up today (thought I wasn't gonna be able to do it till Monday) and installed the game on it. I haven't noticed anything being slower in build/buy than before, though I didn't have time to play that much yet.

Anyway leaving the folder in its place worked just the same so thanks! :)

Strange. What kind of install is this -- Origin, retail store discs with or without Origin, something else? What is the custom destination?