My MacBookPro will not backup to new external drive (two of them). It just stays a PREPARING BACKUP?

How are the backup drives formatted? Drives should use JHFS+.

Drive Partition and Format - El Capitan or Later

1. Select Disk Utility from the Utility Menu and click on the Continue button.
2. After Disk Utility loads select the target drive (out-dented entry with the mfgr.'s ID and size) from the side list. 
3. Click on the Erase button in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply/Erase button and click on the Done button when it is activated.
6. Quit Disk Utility.

Preparing backup may take quite some time as will the first backup.

Back up your files with Time Machine on Mac - Apple Support

Types of disks you can use with Time Machine on Mac - Apple Support

Restore items backed up with Time Machine on Mac - Apple Support

How to use Time Machine to back up or restore your Macs

macOS User Guide