Are you Condensing or New File?
"but then we plan to condense our company file from 12/31/17 back."
I would never do this; I would keep at least the most recent 3-5 years in detail. You will lose a Lot of reporting ability, when you condense. If you have any jobs ongoing across the year end, you will make a bit of a mess with Condense to that most recent year end, as well.
"I am wondering if there are any glitches with that plan. Does anyone know if we will have issues opening up our back up from the external hard drive? And is there a way to keep detail for like 1/1/15 to 12/31/17 and condense prior to 1/1/15?"
You set the date for the condense; you can also Delete everything from a specific date, back, and then repopulate the missing Balance Sheet data.
Starting a new file might take .5 - 2 hours to get up and running, then you will need to start making data entries (transactions) and you build on the complexity as you go. This is a Transition task, as if you are new to using QB. I have only recommended a new file once or twice on 20 years of using and supporting QB.
The majority of my clients use the Job Tracking functions. Unless you consider your file structure, chart of accounts and items list a Friggin' mess that cnanot be resolved, starting a new file is sort of like a frontal lobotomy for your financial tracking and reporting.
