I’m glad Christopher added the reference to his blog – his advice, along with Greg Jones’ many postings in this forum are the best things to read about Indexing.
All of these questions have been answered in the forum, and I suggest you do some investigation here. You’ll find lots of folks have spent a lot of time developing detailed advice – such as this Import vs index (again)
“How quickLy” depends on how much data you have, and how much free space you have. I have 10GB of databases in my work folder and that was collected or created over 5 years. No where near full. This is really an impossible question to answer. Just be aware it isn’t DEVONthink that creates data – it is you.
Imported documents are stored inside the database. The database is a type of folder that OS X calls a “package”. If you control-click a database package (files with .dtBase2 extensions) in Finder and choose “show package contents” you’ll see a hierarchy of folders inside. (Look; don’t touch. Only DEVONthink should touch the contents of a database.)
One of these is “Files.noindex” and all of that folder’s subfolders contain your data. The data files themselves are untouched – DEVONthink NEVER changes your data files. But they are stored in what appears to be an obscurely-named set of folders. That’s just so DEVONthink knows where to locate (in an instant) what you’re looking for. You can always drag or export individual files or groups from a database using the DEVONthink program – or you can use DEVONthink to export the whole hierarchy of folders and documents (100% of the database contents) intact to an external drive or somewhere on your computer’s drive.
The other files in the database package with names ending in “.dtMeta” are used by DEVONthink to make Search and See Also & Classify, and other features, possible. They contain data about your documents, but these files do NOT contain your documents. Metadata files take up a minor amount of space so they are not going to fill up your hard drive. (E.g., in a 2000 MB database I have the metadata files take up 110 MB – 5%).
Your documents are safe inside the database – it might take a wee effort to export them, but if didn’t have the structure that DEVONthink uses inside the database then you wouldn’t have the benefits of the program you bought. The good news is that the same benefits that DEVONthink provides with imported data in terms and search and AI, you’ll get with indexed files.
I do a similar thing – I have a Dropbox folder with hundreds of manuals, tip sheets, etc.
If your manuals’ library is inside a root folder (say a folder creatively-named “Manuals”) and you index Manuals, then every child folder and document inside Manuals comes along too and is accessible in DEVONthink. As long as you do the reorg within the children of Manuals you’ll be ok. The alternative, moving something out of Manuals, is also possible – and I’ll tell you again to look for all of Greg Jones’ advice on how to do this. Search the forum – the topic is covered frequently and in depth.
You can index documents in Dropbox (most of my indexed documents are there) but NEVER put the database itself in Dropbox unless you want to see Dropbox shred it for you. If you need the database on more than one machine, use DEVONthink Sync (built in).
Edit: Here’s some other info that might be useful (sticky topics in the Tips, Tricks & Troubleshooting section of the forum)
Tip: Accessing/Editing Documents Outside of DEVONthink Accessing and Editing Documents Outside of DEVONthink
Tips on Classify & See Also Tips on Classify and See Also
DEVONthink Database Size DEVONthink database size
I also suggest digging into Eric’s blog for lots of relevant tips