I am strongly considering upgrading my Windows Phone 8.1 to the Windows Phone 10 Tech Preview to test some new features, but I'm trying to do some research on the risks associated with doing so before I actually decide to go through with upgrading. By upgrading, as I understand, all data stored in the phone will be erased. Will I need to worry about my SD card being erased?
All your data should be kept intact through the upgrade, though it wouldn't be a bad idea to create a backup first. If you are really worried, your best bet would be to remove your SD card while upgrading.
No, your SD card and phone data will be not erased. But your phone will hang a lot. There are too many bugs in W10 TP.