how do I format a new ext SSD to use for Time Machine backups?

Thanks for the help. It's critical that I not lose what's on my computer until I can first transfer everything to the new ext SSD, then transfer/copy that back from the ext SSD to new int. SSD.

'SanDisk re formatting ext SSD: 

https://kb.sandisk.com/app/answers/detail/a_id/20542/kw/format%20extreme%20portable%20ssd

This is from the SanDisk website. It appears to confirm what you said/your advice, and it also appears to answer my question re APFS.

CAN I FOLLOW THESE DIRECTIONS SAFELY? My understanding is that, after doing this, I will have a properly formatted ext. SanDisk SSD with nothing on it–no files/info/documents etc. on it, and those files/info/documents etc. will still remain on the original int. SSD (Samsunt 860EVO) until the next step of transferring them over to this SanDisk ext. SSD.

Just wanted to thank all of you who helped me thru what for me was a very scary problem (both int/start-up and ext/back-up drives giving failure warnings at the same time) and give you a final update.  I think I got it all done w/o any loss of files or data. Here’s where I’m at:

1) Both my original int & ext drives have been reformatted & now appear to work perfectly. 

2) I put a new WDC 1T SSD in as my main/start-up drive.  I’m using the original Samsung 

500mb SSD (the orig. main/start-up drive) as a second internal drive used simply as a 

“copy” disk (with Super Duper) to keep an actual copy of my main drive (WDC). This way, 

either drive can be used as a start-up disk. 

3) I now have two external back-up disks: 

a) the new SanDisk Extreme Portable 2T SSD;

b) the “old” Seagate 2T HDD.

4) All four drives are formatted Journaled HFS–GUID Partition. Somewhere along the line, I was thrown by an article (think it was Apple.support) that said Mojave uses the Apple File System (APFS), so I thought I needed to convert because it was newer, therefore better.  But not mandatory and, if I’m correct, Mojave must use Journaled in order to work with Time Machine.  

ONE PROBLEM:  While everything now appears to be working well, I still have the original EtreCheck reports showing SSD too slow-SSD is showing poor performance???  Think it might be due to a runaway app–Dictation.  Have posted a new thread titled “Dictation problem. com.apple.SpeechRecognitionCore.speechrecognitiond 82.40 % (Apple)”.

Any help is greatly appreciated.

Terry

Thanks for the help, which helped me solve the problem.  I have one last question re converting to APFS, which I will be posting to the community under the heading: CONVERTING TO APFS.  If you can help, please respond to that post.