Today’s Question: Regarding working with Lightroom on two computers and one external hard drive, can’t you work on the smart previews on either device while not connected to the drive and then sync them when connecting?
Tim’s Quick Answer: Not really. With this type of workflow you would still run into problems with the synchronization of the overall Lightroom workflow.
More Detail: Let’s assume you are working with a Lightroom catalog on your computer’s internal hard drive, with your photos stored on an external hard drive. Even if the external hard drive is disconnected, the standard previews generated by Lightroom would enable you to review your photos, and even update metadata in the Library module. However, in this situation you would not be able to work with your photos in the Develop module until you reconnected the external hard drive.
If you generate smart previews for your photos, however, then you would be able to work in the Develop module even when the external hard drive containing the source images is disconnected. You could even export copies of your photos based on the smart previews. When you reconnect the external hard drive the updates will be synchronized based on the source photos.
However, working with two computers would still be problematic.
Today’s question was in response to a question addressed in a previous edition of the Ask Tim Grey eNewsletter. That question related to the approach of storing your Lightroom catalog on an external hard drive with the source image files, so that you could switch between two computers by simply moving the external hard drive between those two computers.
However, this approach does not enable you to work with smart previews on both computers interchangeably. The smart previews are connected to the actual catalog files, and so you would only have access to the smart previews if you had access to the catalog. In other words, you would need to have the external hard drive connected to the computer in order to access the catalog, which in turn means the external hard drive needs to be connected in order to access the smart previews. However, in this scenario the source images would also be available, so you would not derive a significant benefit from the smart previews (other than a potential performance benefit based on prioritizing the use of the smart previews).
Lightroom does not inherently support working with the same catalog across multiple computers. You can move a catalog between computers (such as by keeping the catalog on an external hard drive) to work around this. It is also possible to use an online synchronization service such as DropBox (https://www.dropbox.com) to enable a workflow across two or more computers. However, there are also some inherent risks with this approach, so it is not something I would generally recommend.