Remove SD-Card + HDD = risk?

Hello there!

I´m new to Linux, NextCloud, etc. and have a question:

I´m using a Raspberry Pi 4 with NextCloud installed on Ubuntu on a SD-Card. I have plugged in a HDD, where all the data is stored. Now I want to use the Pi for another little Halloween-Project.
My current thoghts:

  1. Shutdown the Pi
  2. Remove the SD-Card and unplug the HDD
  3. Insert another SD-Card with a different OS
  4. Halloween-Project
  5. Swap the SD-Card back and plug in the HDD
  6. Use the NextCloud

Can anything go wrong with this approach, for example data loss, or is everything safe so I can end up using the NextCloud as before?

Thank you!

Perfectly fine if you are using an entirely different disk / card.

Thank you very much!