How to get macOS High Sierra file vault encryption to continue/How to cancel encryption and decrypt drive?
You can check the status from the terminal with diskutil cs info /Volumes/&lt;name&gt; and look for Conversion State and LV Conversion Progress. If you want to cancel the encryption with diskutil cs revert /Volumes/&lt;name&gt;

srv:~ onik$ diskutil cs info /Volumes/OSX
Core Storage Properties:
   Role:                       Logical Volume (LV)
   UUID:                       00000000-0000-0000-0000-000000000000
   Parent LVF UUID:            00000000-0000-0000-0000-000000000000
   Parent LVG UUID:            00000000-0000-0000-0000-000000000000
   Device Identifier:          disk1
   LV Status:                  Online
   Conversion State:           NoConversion
   LV Conversion Progress:     100%
   Content Hint:               Apple_HFS
   LV Name:                    OSX
   Volume Name:                OSX
   LV Size:                    126804688896 B


Oh, and if you're using APFS (new in High Sierra), the commands are diskutil apfs list and diskutil apfs decryptVolume /Volumes/&lt;name&gt;

Then you can wait  15 minutes and check again. The conversion should continue and might go faster if you log out or just restart your machine and don’t log back in.

There’s no need to reinstall the operating system in almost all cases. You may need to leave the machine running for a day or two if for some reason it’s not progressing at a good rate. There also is no cancel. It needs to proceed or you could wipe the entire disk and reinstall everything.