The preview cache can be rebuilt as needed with no loss of data.
Since the photos are on an external drive, why not put the LR catalog on the same external drive? Thus no file sync needed.
If you can get the dropbox sync to do the catalog without doing the preview cache, this would be the best option (minimum file transfers with no deleting).
Because the external isn't SSD and the internal is SSD and it reads and writes around 800. So it can rebuild my entire lightroom previews extremely quickly and I'm able to work really fast. So basically, the previews don't matter, just keep doing what I'm doing?
I can't find anyway in dropbox to get it to just sync the catalog but not sync the previews.
Alternatively, if the catalog and previews are on the external drive, you don't need to keep rebuilding the previews; they are already there.
I thought that might be the case with dropbox. Instead of placing your catalog in the dropbox-sync folder, you would need to place it elsewhere and copy the catalog from and to the dropbox-sync folder before and after you use the catalog.
Yeah, but the problem is, if my catalog is on my external it's not going to be as fast.... At least that is what people have told me. I have done speed test. I have extremely fast external SSD drives, I also have really fast 7200RPM portable and desktop drives. None the less, they aren't as fast as my internal SSD on both of my Macs, which are 512GB.
I haven't decided what to do, even now. I'm working off of just one computer, but I can't decide once I start my main catalog. All my previous work I put in a previous catalog. Since that time I have not taken any pro photos that I want in the main catalog that I'm starting for this year onto next May. I need to find out a workflow fast.
On my last Catalog Dropbox worked well for me. I even synced the previews. However, due to my 20GB a month cap at one location, which is mostly where I stay, I noticed the bigger the catalog the bigger the previews get. But Dropbox allowed me to have my catalog on my other macbook pro when I'm at my other home which has fiber optic unlimited internet. The download speed is as fast as the upload speed.
At my other house, they don't run internet out there. So, I have to use Verizon 4G LTE. It gets about 30 down and 5 up. So its fast but I have to be careful and not hit 20GB. This month I have 40GB. Sometimes I change it when I'm gonna be doing some heavy stuff.
At one time I had 100GB when they had a sale going on, 100GB for the price of 50GB. But it got way too expensive. I could afford an apartment with the amount of money I pay for internet. The ping time is fast and it works great but it's expensive. Very expensive.
20GB is more than it sounds like, as long as I watch youtube at that location in 360p or 480p. I don't do Netflix. Other than that I just do regular surfing and uploading photos and using dropbox. I may have to just start bringing both mac laptops with me when I go to my other home so I can upload there. I just wish I could get it down to only bringing one.
The macbook at my location that has 20GB a month is used as a desktop. I have two monitors, Apple thunderbolt, keyboard and one of those huge trackpads. The laptop is closed, so everyone thinks it's a desktop. This is mostly because I like having the 27 inch screens. I only use it as a laptop when I have to upload something from another location to sync.
I have thought about moving the catalog to the slim retina macbook pro which I use as a laptop from a flash drive or 500gb portable drive or something. That would use no internet. Dropout is just so much easier though. Also, I work with people who do the same hobby and I'm able to open up my Lightroom catalog on their copy of Lightroom and show them all of my pictures, because the pictures are on an external 7200RPM drive. They are also on a desktop drive but I don't carry that around. That's more for having 4TB of images. I carry around 2 TB of images.
Dropbox has been the best solution I've found. Also, on my iPad and iPhone, I don't have to use up it's internal storage. All of my picture are on dropbox and videos and movies. So I can stream them from anywhere, show my pictures to anyone anywhere and not have to sync all the time. I have a 128GB iPhone 6 Plus and 128GB iPad mini 3 and both of them have hardly no photos on them, due to dropbox. The only thing I load on the iPad is movies.
The 128GB iPhone 6 plus is used to record song ideas. I have a Rode i-XY condenser mic on it. I also have a Rode SmartLav+ so i can use it for a wireless lav mic.
I know I ranted too long there... Just trying to stress how good dropbox is for someone like me.
True, if the catalog is on the external drive then it is not going to be "as fast" as the internal SSD drives. You would need to decide if the external drive is "fast enough".
One way to go with Dropbox would be to upload and download manually by copying the catalog to and from the Dropbox sync directory as needed, instead of using the catalog in the Dropbox directory.
If you are willing to do the Dropbox sync in a more manual way, you could also do that more manual approach with copying the catalog to and from the external drive.
Syncing a catalog between two computers does take some thought in weighing the ease vs cost of the various methods.