Hard to tell without knowing how much footage you have. The WD Mybook is a consumer drive, but it will work for editing if it's USB3 (which I believe they all are now). USB 2 won't be sufficient. Bear in mind that you should also leave 15% to 20% of free space on any drive to allow for drive management and better efficiency, so with a 6TB drive that means roughly 1.2 TB of unused space. So, the size really depends on how much footage you wish to edit and store on that drive. You might check OWC and G-Tech drives for more reliable options depending on your budget.
For traveling with external hard drives, I would suggest sticking with SSDs (for durability) so you wouldn't have to worry about rpm. I use a Samsung external SSD but there are plenty of good manufacturers out there. Just do a little research and read the reviews. Good luck!
I have been having problems lately with hard drives that I have edited FCPX projects on. My current thoughts on this subject are to do the following.
1) Store source files on two external hard drives. Let one be the duplicate backup of the other. And DO NOT edit FCPX projects off of these drives or use them as a target for files to be rendered by FCPX or Compressor.
2) Do all editing on projects stored on external SSD drives. 500 GB SSD drives seem to be at the sweet spot right now for cost, performance, and storage capacity. Use proxy files to keep project file sizes down. Use a separate external drive to store backups of your projects. Store archived projects (sans FCPX generated files) on the drives from 1) (just don't edit them later from those drives-put them on the SSD you want to re-edit). Or store the on separate duplicate hard drives used for archiving FCPX projects.
3) Export final rendered files to 128 GB flash drives, such as the SanDisk ones sold inexpensively at Costco. Format these in APFS, NFS+, or exFAT to avoid the 4 GB file-size limit of FAT32, which is the file system these drives are typically formatted in when new.
You might want to consider giving drives FCPX projects are edited off of an "expiry date" (perhaps two years) before acquiring a new one for editing. FCPX projects will involve a lot of reads and writes, and causes a fair amount of wear and tear.