I wish to update the info in the previous post.
I have 50K photos on a 2T LaCie RAID 1 drive, along with a buch of other stuff. Disk is mounted and readable/writable from my Macbook Pro. The first catalog created with Add was place on the Macbook internal 256GB SSD. (I never got the procedure in the original post to work). I next tried to export a catalog to the LaCie disk. Same problem as above: LR only showed the device greyed out and I couldn't select it. Next I exited LR, restarted, creating a new catalog on the LaCie drive. No problem. Next I restarted LR again, this time selecting the original populated catalog. Now I try the 'export catalog' again, and magically it works. The LaCie drive properly appears and I can select it.
Pretty sure there's no pilot error here. Since other apps have no problem, I suspect LR has a bug, but I don't have incentive to try debugging it any further at this point.
Are you trying to export the catalog and the images, by selecting the option to include negs? If you are doing this, there may not be enough room on the external drive.
No negs. There's more than enough room -- 700+GB. The 50K Images are around 150GB.
Moreover, the problem occurs when trying to select the media for output--it remains greyed out and couldn't be selected until I went through the procedure above.