The current bootable HD will be the main HD and still boot. When adding the 2nd old HD, if need be, claim ownership. If you have no need for any data from the old HD, you can format it and then prepare for its use. HOWEVER, the safest way to do this is to connect the old HD alone. Disconnect the new PC HD, entirely(no need to remove it). Attach the old HD and using either freebie KILLDISK or DBAN pgm. wipe out/clean the old HD. Once done, you can return the new HD and old HD into PC. The old HD needs to be formatted, do so. Use the disc management tools under the administrative tools area. Amke darn sure what HD you're working on. Does this answer your question?

BTW, what does your new PC consist of, because it should be able to handle larger than 500gb HD installed. Something seems amiss here.

tada -----Willy Happy